ARTICLE pubs.acs.org/Langmuir
Fluorescence-Based Aluminum Ion Sensing Using a Surface-Functionalized Microstructured Optical Fiber Stephen C. Warren-Smith, Sabrina Heng,* Heike Ebendorff-Heidepriem, Andrew D. Abell, and Tanya M. Monro Institute for Photonics & Advanced Sensing, School of Chemistry & Physics, The University of Adelaide, Adelaide, South Australia 5005
bS Supporting Information ABSTRACT: The first microstructured optical fiber-based sensor platform for aluminum ions using a surface-attached derivative of lumogallion (3), a known fluorescence-based indicator, has been fabricated. These fibers allow for strong evanescent field interactions with the surrounding media because of the small core size while also providing the potential for real-time and distributed measurements. The fluorescence response to aluminum ions was first demonstrated by applying the procedure to glass slides. This was achieved through the covalent attachment of the fluorophore to a polyelectrolyte-coated glass surface and then to the internal holes of a suspended-core microstructured optical fiber to give an effective aluminum sensor. Whereas the sensor platform reported is fabricated for aluminum, the approach is versatile, with applicability to the detection of other ions.
1. INTRODUCTION Existing staining-based methodology1,2 for the detection of metal ions is not well suited for real-time or in situ use. This is a significant problem, given that these ions can have a considerable impact on both human health and the environment. For example, aluminum, copper, iron, and zinc have been linked to Alzheimer’s disease,35 where they are thought to influence the formation of amyloid fibrils.1 In agriculture, the release of aluminum ions from acidic soils is also known to be toxic to plant roots.1,2,6,7 The presence of aluminum ions is also an important marker for monitoring the corrosion of aluminum alloys2,3 where current ways of detecting corrosion are simply carried out via visual inspection or weight monitoring.4 New methodology is required if we are to detect aluminum and other metal ions rapidly and efficiently. Although optical fiber-based aluminum sensors have been reported,5 these reports are limited to extrinsic distal end measurements.3,6 Microstructured optical fibers (MOFs) offer the potential to improve performance relative to more traditional spectroscopic and fluorescence-based fiber sensors.13,14 For example, the use of MOFs is significant because the air holes running their length1517 enable light to be guided in the fiber’s core either through total internal reflection or bandgap effects.7 The holes within the MOFs act as tiny sample chambers to allow surface functionalization and local chemical reactions. In addition, these holes can be used to control the interactions between the guided light and any matter located within the holes; manipulating the geometry of the fiber cross section can increase the fraction of the guided light that is available to interact with the material located within the holes relative to conventional optical fibers.8 Thus, the appropriate design of the cross-sectional structure of the MOF r 2011 American Chemical Society
can provide the broad range of optical properties demanded by sensors.7 Finally, MOFs also provide immunity to potential electromagnetic interference, with the added advantage of being flexible and lightweight enough for use in confined and inaccessible spaces. In this article, we demonstrate the potential of an MOF-based sensor in the detection of aluminum ions. We use a small-core suspended-core fiber because these are relatively simple to fabricate9 and fill,10 while providing high-efficiency fluorescence sensing11 using small (nanoliter-scale) sample volumes.12 This new sensor platform uses an intrinsic, evanescent wave sensor that utilizes the interaction length possible with optical fibers. To use the MOFs as aluminum sensors, the fiber must first be functionalized with a fluorescent indicator designed with the following points in mind. First, the indicator must be chemically suitable for surface attachment onto the glass and fiber surfaces functionalized with a polyelectrolyte layer.13,14 Second, its fluorescence must increase when complexed to aluminum because quenching-based systems suffer from false positive detection as a result of factors such as fiber degradation, photobleaching, and reduced coupling efficiency. Third, the operating wavelength should be as long as possible because the transmission loss of the F2 leadsilicate glass used in the fibers is lowest between 500 and 1500 nm. Thus, the indicator molecule must operate in this range for distributed sensing to be feasible over reasonable length scales. Finally, the fluorophore must possess high chemical and photostability because the sensor should be operational for long Received: January 19, 2011 Revised: March 6, 2011 Published: April 06, 2011 5680
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Scheme 1
periods of use. Here we present the first example of an MOF internally modified with such a fluorescent indicator to allow chemical sensing. The resulting sensor offers advantages of the rapid in situ measurement of aluminum ions with low sample volumes and the possibility for spatially distributed measurements while also demonstrating a new platform for use in other chemical sensing applications.
2. EXPERIMENTAL SECTION 2.1. Chemical Synthesis. The novel lumogallion derivative (3) was synthesized in a yield of 95% by reacting 4-amino-3-hydroxybenzoic acid with resorcinol in the presence of sodium nitrate as described for related compounds (Scheme 1).15 2.2. Cuvette Measurements. Lumogallion was first dissolved into a pH 5 acetic acid buffer solution. All cuvette samples were prepared without any further purification or modification at room temperature using distilled water as the solvent. Disposable plastic 1 cm 1 cm 4.5 cm cuvettes were used for all cuvette measurements. Samples were excited using a 532 nm laser (JDSU, solid state, compact), and spectra were recorded using a fiber-coupled cuvette holder (Ocean Optics, CUV-ALL-UV) and a fiber-coupled spectrometer (Ocean Optics, QE65000). 2.3. Glass Slide Experiments. 2.3.1. Preparation of Glass Slides. Eight glass slides were prepared by cutting standard microscope slides into dimensions of 1 cm 2.5 cm using a diamond saw. The slides were then rinsed thoroughly with distilled water and methanol and then dried with nitrogen gas. 2.3.2. Preparation of Coating Materials. Poly(allylamine hydrochloride) (PAH, Mw ≈ 56 000) was purchased from Sigma-Aldrich. The 2 mg/mL solution was prepared by dissolving 200 mg of PAH in 100 mL of 1 M NaCl. The 100 μM solution was prepared by dissolving 5.5 mg of the lumogallion derivative in 200 mL of distilled water. Then, 200 μL of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and Nhydroxysuccinimide (NHS) solution (Biacore coupling solutions) was added to 60 mL of the lumogallion solution, of which approximately 30 mL was used for the experiment. 2.3.3. Fluorescence Imaging Settings. The settings used for the Typhoon variable-mode imager (8600) were an excitation wavelength of 532 nm, a 560 nm long-pass filter, normal sensitivity, a resolution of 200 μm, and a photomultiplier tube (PMT) voltage of 600 V. 2.4. Microstructured Optical Fiber Coating Procedure. The first step in the coating procedure was to flush PAH solution through 2 60 cm lengths of wagon wheel (WW) fiber with 20 psi pressure for 3 h. To apply the pressure, the fibers were sealed into a metal chamber using
Figure 1. Cuvette fluorescence measurements for the commercially available lumogallion (1, 50 μM) and the lumogallion derivative (3, 50 μM), which both show an increase in the fluorescence intensity in response to the presence of aluminum ions. a rubber seal and regulated nitrogen gas was fed into the chamber using a pressurized nitrogen gas bottle. The fibers were then flushed with water for 30 min at 40 psi and then with nitrogen for 30 min at 40 psi. Stock solutions of compound 3 (50 μM in distilled water) and EDC and NHS (1 M each) were prepared. Next, 200 μL of EDC and NHS was added to 10 mL of the solution comprising compound 3, of which approximately 2 mL of the reaction mixture was used for the experiment. The fibers were then flushed with the reaction mixture for 3 h at 20 psi and then rinsed with distilled water and nitrogen as done after the PAH coating. Note that the fibers were cleaved at both ends after each step to prevent residue from blocking the holes. Also, the whole length of the fiber was checked under an optical microscope after each nitrogen-flushing step to ensure that all liquid was removed from the fiber.
3. RESULTS AND DISCUSSION The fluorescent indicator (3), used for surface attachment and sensor development, was based on lumogallion (1), which is readily known to form a fluorescent complex with aluminum.2,16 The resulting complex has excitation and emission wavelengths of 500 and 570 nm, respectively. This derivative contains a carboxyl moiety in place of the sulfonic acid of lumogallion to allow ready attachment to a glass surface. It also lacks the chloro substituent of lumogallion to improve the hydrophilicity of the molecule. The reported limit for detecting aluminum using lumogallion in solution is on the order of μg L1.17 Although lumogallion is known to bind gallium and indium ions in addition to Al3þ ions,18 this lack of specificity is not a significant issue for applications such as corrosion sensing because the detection of other metal ions in solution would also likely correspond to corrosion. The same can be said in the case of Alzheimer’s disease, where interference from the presence of Ga3þ and In3þ ions is unlikely. In addition, lumogallion is not known to bind to other biologically relevant ions such as Ca2þ, Mg2þ, and Zn2þ.19 In any event, the purpose of the current study is to demonstrate the feasibility of the approach rather than to produce an optimized sensor. The fluorescence properties of synthesized 3 were first compared to those of lumogallion (1) in solution to ascertain whether it was suitable for sensor development. The fluorescence spectra are shown in Figure 1 for samples with concentrations of 5681
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Figure 2. Integration of the cuvette measurements where the dark noise has been removed and the results have been divided by the 0 μM aluminum ion measurement. (b) Data and linear fit of lumogallion (1) in the presence of Al3þ where R2 = 0.98. (Red b) Data and linear fit of compound 3 in the presence of Al3þ where R2 = 0.99. This shows that 3, unlike 1, emits fluorescence that increases linearly with increasing aluminum concentration. Identical results were obtained for all of the different aluminum concentrations tested.
Figure 3. (a) Schematic showing the absorption of the polycation onto the glass slide. Shown also is the chemical structure of the polycation used in this experiment. Counter ions are omitted for simplicity. (b) Simplified representation depicting the product of coupling the lumogallion derivative (3) with the PAH layer.
0 and 100 μM aluminum ions present and a lumogallion concentration of 50 μM. Figure 1 shows that the fluorescence of 3 is comparable to but slightly lower than that of 1. However, this slight decrease occurs for both complexed and uncomplexed forms of the compound and is thus not necessarily disadvantageous when competition with the uncomplexed lumogallion background is the limiting factor in the measurement. An inspection of the spectra that have been integrated and normalized to the 0 M Al3þ result (Figure 2) shows that the lumogallion derivative (3) emits fluorescence that increases linearly with increasing aluminum concentration, making it suitable for a sensing application. In this experiment, the minimum concentration of
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Figure 4. Here, the lumogallion derivative refers to compound 3. (Top) Order of the control slides. Drops of aluminum ions were placed onto the glass slides. B refers to a drop of the background solution (water); 10 = 10 μM Al3þ, 100 = 100 μM Al3þ, and 1m = 1 mM Al3þ. The first four slides are negative controls. (Bottom) Fluorescence imaging results 45 min after placing the aluminum ion drops.
Al3þ that could be measured was approximately 500 nM. Though an Al3þ concentration of as low as 200 nM is measured, the signalto-background ratios obtained for the 200 nM sample were only 1.6 and 1.9 for compounds 3 and 1, respectively, as compared to the 2.6 and 3.3 signal-to-background ratios obtained for 500 nM samples of compounds 3 and 1, respectively, which is a more reliable measurement. Compound 3 was then attached to a glass slide as a first step towards a surface-functionalized microstructured optical fiber. Although there are some differences in the chemical composition of standard silica microscope slides and the MOF that is subsequently used, which is fabricated from leadsilicate (F2) glass, both are silicate glasses with wettable (hydrophilic) surfaces and thus this experiment provides a valuable comparison and model system. Our aim was to develop surface-attachment methodology compatible with an MOF-based approach for the detection of a range of ions using a range of fluorophores. In this article, we demonstrate the first application of such an approach with the detection of aluminum ions. The confined nature of the holes within the MOF cross section dictates that localized surface techniques must be used.13,20 With this in mind, we specifically chose to use a polyelectrolyte-based system to functionalize the glass surface electrostatically to provide a base layer for covalently attaching 3 (Figure 3).20 The advantage of this method is that, because it is a physical and not a chemical interaction, it can be applied to a range of surfaces, including different glasses.21 In addition, polyelectrolytes can provide a greater number of binding sites and a lower optical loss compared to silanization methods.21 Eight standard microscope slides were first coated with a single electropositive polyelectrolyte layer (Figure 3a). Fluorophore 3 was subsequently coupled onto the polyelectrolyte layer using standard coupling conditions (Figure 3b). The slides were then rinsed thoroughly with distilled water and dried with nitrogen gas. Controls were prepared containing layers of only PAH or 3, with the order of these controls shown in Figure 4. The fluorescence response of the coated slides to the presence of aluminum ions was then measured using the fluorescence imager. The glass slides were placed onto the imager surface, and drops of the aqueous aluminum(III) potassium sulfate 5682
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Figure 5. Suspended core microstructured optical fiber made from F2 glass.
Figure 7. Integration of resultant fluorescence before and after filling of the fiber with compound 3. Light (532 nm) was coupled using a 100 microscope objective and a dichroic mirror.
Figure 6. Optical setup used to measure the fluorescence from the MOF when filled with an aluminum ion solution. MMF is a multimode fiber, and LP is a long-pass filter. The 633 nm laser was used only for alignment in the spectrometer and was turned off prior to measurements. The fibers were filled with aluminum ion solutions by immersing the right-hand-side end of the fiber and allowing it to fill via capillary forces.
(AlK(SO4)2 3 12H2O) solution were placed onto the slides in the arrangement shown in Figure 4. The slides were then scanned after 45 min with the results shown in the lower section of Figure 4. The data reveals a strong correlation between the aluminum ion concentration and fluorescence intensity, and the control slides show a negligible response. These results confirmed the binding of compound 3 to PAH and that the fluorescence response of the fluorophore is retained in its immobilized state, which demonstrates the applicability of the approach for aluminum ion sensing. However, note that experiments using pH 5 buffered aluminum ion solutions (acetic acid/sodium acetate buffer) demonstrated that the coating was easily removed (results not shown here). Thus, all glass slide experiments and fiber experiments reported here were performed using distilled water as the only solvent. A possible explanation is that acetic acid forms a salt with the PAH layer, allowing it to wash off of the glass surface. Thus, although both lumogallion (1) and compound 3 operated successfully between pH 3 and 7 (cuvette experiments) and lumogallion generally has a wide pH operating range from 2.0 to 5.7,22 further optimization needs to be carried out in order to improve the durability of the polyelectrolyte coating. The approach was then applied to an optical fiber sensing platform using a suspended-core F2 fiber with a core diameter of approximately 1.7 μm and three holes with diameters of approximately 12 μm (Figure 5). The fiber was fabricated in-house9 using active pressurization of the fiber holes during fiber drawing in order to increase the hole diameter and thus improve liquid filling speeds and reduce hole blockages. Previous hole diameters of noninflated fibers with similar core diameters were approximately 6 μm.9 However, these smaller-diameter holes can become blocked when filled with coating materials, thus the larger-holediameter fibers in Figure 5 were used in this work.
Figure 8. Fluorescence counts for a surface-functionalized aluminum sensor based on a suspended-core F2 fiber. Each plot corresponds to a different section of fiber that was tested where four separate pieces were used. Black curves were recorded prior to filling the fibers with an aluminum ion solution and correspond to the background fluorescence of the coating materials. Orange curves were recorded 10 min after filling the fibers with either a 10 mM aluminum ion solution (upper plots) or a 30 mM aluminum ion solution (lower plots).
The coating solutions were forced through the fiber holes under a positive pressure of nitrogen gas developed by sealing the fibers within a metal chamber using a rubber seal. The fibers were first flushed with the PAH solution for 3 h and then rinsed with distilled water and nitrogen for 30 min each. The fibers were finally flushed with a solution of 3 for 3 h, followed by rinsing with distilled water and nitrogen for 30 min. The resulting fibers were subsequently cleaved into 6 cm pieces and placed into the setup shown in Figure 6 for fluorescence measurements. The fibers were filled with 10 μM, 100 μM, 1 mM, and 10 mM aluminum(III) potassium sulfate solutions for 10 min using only capillary forces, and spectra were recorded after another 10 min. Background spectra were also recorded prior to filling. The spectra taken before and after filling were 5683
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Langmuir subsequently integrated, and the final results are shown in Figure 7. The results in Figure 7 reveal a significant fluorescence response, over the background, down to a concentration of 100 μM Al3þ ions. The apparent 10-fold decrease in sensitivity compared to the glass slide experiments (see discussion above) is due to the large background fluorescence from the uncomplexed fluorophore and not from the fiber itself. This is apparent when comparing the results shown for “uncoated, water” and “coated, water” (Figure 7), where a significant increase in background fluorescence was observed after coating. This decrease in sensitivity is attributable to the high surface density of the fluorophore on the fiber, which is common in surfaceattachments experiments.10,23,24 Figure 8 shows the response of the probe toward two different concentrations of aluminum with some slight variation between different sections of the fiber samples. This variability arises, at least in part, from difficulties in reproducibly coupling the fluorophore onto the small core fibers, which might be overcome using fiber splices.12 The results in Figure 8 demonstrate the current detection limit of the optical sensor, for which the maximum fluorescence intensity relative to the background fluorescence can be obtained. The 10 mM samples (Figure 8a,b) show an increase in the fluorescence signal after filling with the aluminum ion solution. A significantly larger increase in fluorescence is apparent for the 30 mM sample (Figure 8c,d), where the background fluorescence becomes negligible. We thus demonstrate for the first time that it is possible to detect Al3þ ions using an MOF-based sensor. The challenge now is to increase the sensitivity.
4. CONCLUSIONS The study presents the first surface-functionalized microstructured optical fiber-based chemical sensor. Our approach is demonstrated for the detection of Al3þ ions using a modified lumogallion fluorophore (3), but it is directly applicable to the detection of other ions, particularly Ga3þ and In3þ ions to which lumogallion is also known to bind. The development of other fluorophores will allow the application of our new sensor platform to the detection of other ions. We also demonstrate the potential of polyelectrolytes in the electrostatic attachment of fluorophore (3) to the fiber surface. The fluorophore (3) has been shown to be sensitive to Al3þ ions in solution at a concentration of 1 μM. Significant fluorescence was also observed for 3 attached to a glass slide in response to 10 μM Al3þ ions. The attachment of 3 to an optical fiber via the PAH layer produced a photostable surface that was able to detect Al3þ ions at a concentration of 100 μM. Although this MOF-based system is less sensitive than the glass slide system (10 μM) and that of other reported conventional (not MOF) optical fiber aluminum sensors6 (20 nM), we have demonstrated the potential of the approach. The apparent decrease in sensitivity is attributable to the high surface density of the fluorophore, which is a common issue with surface-attachment experiments. In addition, our group has recently demonstrated improvements to fluorescence sensing in soft-glass MOF that result in significantly improved sensitivity relative to that in previously published results. In this new work, the detection of concentrations of CdSe quantum dots down to 10 pM levels has been demonstrated.25 This was achieved using the same fiber concept, thus paving the way for the optimization of our approach and its
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use in important applications, such as the real-time, in situ sensing of aluminum ions for biological applications and in corrosion-sensing applications.
’ ASSOCIATED CONTENT
bS
Synthetic procedure and 1H and C NMR data for compound 3. Cuvette preparation, glass slide coating and fluorescence imaging procedures, and an optical fiber coating procedure. This material is available free of charge via the Internet at http://pubs.acs.org. Supporting Information.
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We acknowledge Markus Pietsch for early work on the chemical attachment concept, Roger Moore for fabricating the inflated suspended-core fibers, Alexandre Francois for the suggestion of using polyelectrolytes, and Peter Hoffmann for providing the Typhoon Imager. This work was supported in part by the Defence Science and Technology Organisation (DSTO) Corporate Enabling Research Program in Signatures, Materials and Energy and the Australian Research Council under grants DP0880436, DP0771901, and DP0985176. T.M.M. acknowledges the support of an ARC Federation Fellowship. ’ REFERENCES (1) House, E.; Collingwood, J.; Khan, A.; Korchazkina, O.; Berthon, G.; Exley, C. J. Alzheimer’s Dis. 2004, 6, 291–301. (2) Taylor, S. R.; Chambers, B. D. Corros. Sci. 2007, 49, 1584. (3) McAdam, G.; Newman, P. J.; McKenzie, I.; Davis, C.; Hinton, B. R. W. Struct. Health Monit. 2005, 4, 47. (4) Fuhr, P. L.; Huston, D. R. Smart Mater. Struct. 1998, 7, 217. (5) Potyrailo, R. A.; Hobbs, S. E.; Hieftje, G. M. Anal. Chem. 1998, 70, 1639–45. (6) Ahmad, M.; Narayanaswamy, R. Sens. Actuators, B 2002, 81, 259. (7) Laegsgaard, J.; Bjarklev, A. J. Am. Ceram. Soc. 2006, 89, 2. (8) Monro, T. M.; Belardi, W.; Furusawa, K.; Baggett, J. C.; Broderick, N. G. R.; Richardson, D. Meas. Sci. Technol. 2001, 131, 2925. (9) Ebendorff-Heidepriem, H.; Warren-Smith, S. C.; Monro, T. M. Opt. Express 2009, 17, 2646–57. (10) Ruan, Y.; Schartner, E. P.; Ebendorff-Heidepriem, H.; Hoffmann, P.; Monro, T. M. Opt. Express 2007, 15, 17819–26. (11) Warren-Smith, S. C.; Afshar, S.; Monro, T. M. Opt. Express 2010, 18, 9474–85. (12) Monro, T. M.; Warren-Smith, S.; Schartner, E. P.; Franc- ois, A.; Heng, S.; Ebendorff-Heidepriem, H.; Afshar, V, S. Opt. Fiber Technol. 2101, 16, 343–356. (13) Francois, A.; Krishnamoorthy, S.; Himmelhaus, M. Proc. SPIE 2008, 6862, 686211. (14) Francois, A.; Ebendorff-Heidepriem, H.; Monro, T. M. Optical Fiber Sensing, Edinburgh, UK 2009. (15) Wang, M.; Funabiki, K.; Matsui, M. Dyes Pigm. 2003, 57, 77. (16) Zhang, J.; Xu, H.; Ren, J. L. Anal. Chim. Acta 2000, 405, 31. (17) Nadzhafova, O. Y.; Zaporozhets, O. A.; Rachinska, I. V.; Fedorenko, L. L.; Yusupov, N. Talanta 2005, 67, 767. (18) Carroll, M. K.; Bright, F. V.; Hieftje, G. M. Anal. Chem. 1989, 61, 1768–1772. (19) Wang, Z.; Palacios, M. A.; Anzenbacher, P., Jr. Anal. Chem. 2008, 80, 7451–9. 5684
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(20) Decher, G. In Comprehensive Supramolecular Chemistry; Sauvage, J.-P., Hosseini, M. W., Eds.; Pergamon: New York, 1996; Vol. 9. (21) Foo, T. C.; Francois, A.; Ebendorff-Heidepriem, H.; Sumby, C.; Monro, T. M. Australian Conference On Optical Fiber Technology, Australia 2009. (22) Kataoka, T.; Mori, M.; Nakanishi, T. M.; Matsunoto, S.; Uchiumi, A. J. Plant Res. 1997, 110, 305–309. (23) Lee, H.; Kim, H.-J.; Park, J.-H.; Jeong, D. H.; Lee, S.-K. Meas. Sci. Technol. 2010, 21, 085805. (24) Zhao, S.; Reichert, W. M. Langmuir 1992, 8, 2785–2791. (25) Schartner, E. P.; Ebendorff-Heidepriem, H.; Warren-Smith, S. C.; White, R. T.; Monro, T. M. Sensors 2011, 11, 2961–2971.
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