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Transpiration-Inspired Fabrication of Opal Capillary with Multiple Heterostructures for Multiplex Aptamer-Based Fluorescent Assays Bingbing Gao, Litianyi Tang, Dagan Zhang, Zhuoying Xie, Enben Su, Hong Liu, and Zhongze Gu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10143 • Publication Date (Web): 06 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017

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ACS Applied Materials & Interfaces

Transpiration-Inspired Fabrication of Opal Capillary with Multiple Heterostructures for Multiplex Aptamer-Based Fluorescent Assays Bingbing Gao,& Litianyi Tang,& Dagan Zhang, Zhuoying Xie, Enben Su,# Hong Liu*,†, ‡

† ‡

and Zhongze Gu*, ,

† State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Nanjing 210096, China & These authors contributed equally to this work and should be considered co-first authors

KEYWORDS:

Colloid

Crystal,

Fluorescence

Enhancement,

Transpiration, Capillary

ACS Paragon Plus Environment

Heterostructure,


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ABSTRACT In this work we report a method for the fabrication of opal capillary with multiple heterostructures for aptamer-based assays. The method is inspired by plant transpiration. During the fabrication, monodisperse SiO2 nanoparticles (NPs) self assemble in a glass capillary with the solvent gradually evaporating from the top end of the capillary. By simply changing the colloid solution which wicks through the capillary, multiple heterostructures can be easily prepared inside the capillary. On the surface of the SiO2 NPs, polydopamine is coated for immobilization of aminomethyl-modified aptamers. The aptamers are used for fluorescent detection of adenosine triphosphate (ATP) and thrombin. Owing to fluorescence enhancement effect of the photonic heterstructures, the fluorescent signal for detection is amplified by up to 40-fold. The limit of detection is 32 µM for ATP and 8.1 nM for thrombin, respectively. Therefore, we believe this method is promising for the fabrication of analytical capillary devices for point-of-care testing.


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INTRODUCTION Inspired by the natural creatures, photonic crystals (PCs) are attracting increasing interest in a wide variety of research fields due to their unique optical property and structures.1 Particularly, PCs with controlled micro-/nanostructures have emerged as promising materials for a range of bioanalytical applications.2-5 On one hand, photons with wavelengths near the stopband of PCs propagate at lower speed owing to resonant Bragg scattering, which enhances optical gain leading to stimulated emission as well as amplifies the excitation of incident light.5-8 Based on the fluorescence enhancement effect, PCs have been used to improve the sensitivity for different kinds of fluorescent biological analysis.2, 9-11 On the other hand, the highly-ordered pores of PCs with tailorable surface chemistry constitute an ideal platform for carrying out pump-free capillary microfluidics, as aqueous solutions can wick through spontaneously by capillary action.12

Compared with random pores of cellulose paper used in recently rising

paper fluidics, the highly-ordered pores ensure more uniform flow profile and reproducible analytical results. As a demonstration, we have previously reported a kind of capillary microfluidic device based on PCs of nitrocellulose.11 With the slow light effect, the PCs can enhance the fluorescent detection signal by about 80-fold thus improving the sensitivity of detection. The highly ordered pores ensure uniform flow profile and analytical reproducibility.11 However, the fabrication of nitrocellulose PCs was



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complicated, and only homogeneous photonic structure which consists of pores with the same diameter can be fabricated. To further increase the fluorescent signal, PCs with heterostructures have been investigated.9

Heterostructured PCs13 are composed of two kinds of periodic

photonic structures. This heterostructure has an enormous fluorescent enhancement, which is more obvious than single photonic structure because its stopbands overlap both the excitation and the emission wavelength of the fluorophore resulting in doubly resonant enhancement.9 Moreover, because the time required for liquid to wick through a channel is inversely proportional to pore diameter (Washburn’s equation: L2= γDt/4η),14 the wicking rate can be tuned simply by changing the pore diameter of the photonic channel. Therefore, the heterostructure channel can be potentially utilized to achieve the fluidic control by tuning the pore diameter.11 However, the preparation of PCs with the heterostructure was quite complicated, which involves multiple deposition and coating steps.9, 15 The coating of SiO2 colloid solution on top of another layer may have a negative effect on the structure integrity of the self-assembled SiO2 NPs and the reproducibility of bioanalysis carried out on the material. The fabrication of more complicated heterostructures was still challenging. Inspired by plant transpiration, we developed a method for fabrication of opal capillary with multiple heterostructures for multiplex bioassays. Plant transpiration is a process of water absorption by roots, movement through vessels and its evaporation from aerial parts such as leaves.16 During plant transpiration, mass flow of mineral


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nutrients and water takes place from roots to shoots. We use capillary to mimic the plant vessel and inserts one end of the capillary into the colloidal SiO2 particle solution. Due to the principle of transpiration, the NPs overcome gravity and are concentrated near the top end of the capillary. With solvent evaporation, the NPs self assemble into hexagonal close-packed structure17. By changing the diameter of NPs in the colloid solution, heterogeneous structure can be obtained. The surface of the SiO2 NPs is modified by polymerization of dopamine for immobilization of aminomethyl-modified aptamers to carry out the fluorescent assays. The process of the fabrication is simple, and multiple capillary channels can be prepared simultaneously for high-throughput fabrication. The heterostructure has unique optical properties which are useful for improving the detection sensitivity. The multiple heterostructures in a capillary fabricated using this method lead to fluorescent enhancements for different fluorophores so that it can be used for multiplex tests in a single channel. Therefore, we believe the method for fabrication of photonic capillary represents a significant advance, which will be useful for point-of-care testing.



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EXPERIMENTAL SECTION Chemicals and materials. All silica capillaries (diameter, 500 µm) and ethanol was obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). 3-Hydroxytyramine hydrochloride, ATP and thrombin, 5-carboxyfluorescein (FAM), cyanine 3 fluorescein (Cy3), bovine serum albumin (BSA) was purchased from Sigma-Aldrich. All monodisperse SiO2 NPs with a diameter of 195, 228, 246, 256, 274 and 288 nm (reflection peak: 420, 490, 530, 550, 590 and 620 nm) , respectively, were

synthesized

using

the

Stöber-Fink-Bohn

(5`-AAAAAATGCGGAGGAAGGT-3`), (5`-AAAAAAGGTTGGTGTGGTTGG-3`),

method.18

ATP

thrombin Cy3-labeled

aptamer aptamer

ATP

aptamer

(Cy3-5`-AAAAAAACCTGGGGGAGTAT-3`) and FAM-labeled thrombin aptamer (FAM-5`-AAAAAAAGTCCGTGGTAGGGCAGGTTGGGGT GACT-3`) were all purchased from Genewiz, Inc. (Suzhou, China). 10 µM phosphate-buffered saline (PBS, pH 7.5) and 20 µM PBS (pH 8.5) were used for dopamine polymerization on the surface of NPs and 10 µM PBS (pH 7.5) was used for wash. All solutions were prepared with deionised water (18.0 MΩ cm, Milli-Q Gradient System, Millipore) with ultraviolet sterilization. All reagents were used as received without further purification.


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Figure 1. Schematic illustration showing the procedure to fabricate the opal capillary with multiple heterostructures for enhanced fluorescent aptamer-based assays. (I) The glass capillary was inserted in colloid solution containing SiO2 NPs with a diameter of 246 nm and lift up slowly. With solvent evaporation, the SiO2 NPs self assembled into a uniform layer on both the outer and inner wall of the capillary. The SiO2 NPs on the outer wall were removed by wiping with paper. (II) The capillary was inserted in colloid solution containing SiO2 NPs with a diameter of 228 nm to form upper-half portion of the structure in the capillary with solvent evaporation. (III) The capillary was transferred into colloid solution containing SiO2 NPs with a diameter of 274 nm to form bottom-half portion of the structure in capillary with solvent evaporation. (IV) The opal capillary with multiple heterostructures was prepared. (V) Polydopamine-based surface modification of SiO2 NPs for immobilization of aminomethyl-modified aptamer. (VI) The detection of ATP and thrombin in the aptamer-modified opal capillary with fluorescent enhancement. Experimental Procedures. The fabrication of the opal capillary was showed in Figure 1. First, capillaries (diameter, 500 µm) were cleaned with 0.10 M NaOH, and then rinsed for 20 min with ultrapure water and 100% ethanol. The capillaries were then dried in a vacuum oven at 70 ℃ for 30 min. Each capillary was inserted in a centrifuge tube containing 0.5 mL of 20% (w/v) monodisperse SiO2 NPs in ethanol. The colloid solution wicked through the capillary owing to capillary force. The capillary was tilted occasionally to make sure it was completely filled with the colloid solution. With ethanol evaporation, SiO2 NPs gradually assembled near the top end of the capillary. For fabrication of multiple heterostructured opal capillary, the capillary



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was inserted in 20% (w/v) SiO2 NPs in ethanol and then lifted up to form the outer layer of heterostructure inside the capillary. Next, the bottom end of the capillary was sequentially immersed in two kinds of SiO2 NPs colloid solutions, respectively, and the NPs self assembled to form the multiple heterostructured opal capillary. Polydopamine coating. Polydopamine was coated on the surface SiO2 NPs inside the capillary according to the literature.19-22 Briefly, capillaries were first cleaned using water and then ethanol. Next, the capillaries were dipped into 20 mL aliquot of 10 mM Tris(hydroxymethyl)aminomethane (Tris)-HCl solution (pH 8.5) containing 0.50 mg/mL dopamine, and allowed to react for 12 h. The unreacted were rinsed using water and then ethanol. Aptamer-based assays. One side of the polydopamine-coated capillary with heterostructure prepared using NPs with a diameter of 228，246 and 274 nm, respectively, was soaked in 20 µL of 10 mM Tris-Hcl solution (pH 8.5) containing 1.0 mM ATP aptamer. The capillary was removed from the solution as the solution reached the boundary of the heterostructure and allowed to react at 4 ℃ with a relative humidity of 99% for overnight. After rinsing and drying under ambient environment, the remaining unmodified portion of the capillary was soaked in 20 µL of 10 mM Tris-Hcl solution (pH 8.5) containing 1.0 mM thrombin aptamer and allowed to react at 4 ℃ with a relative humidity of 99% for overnight. After rinsing and drying at ambient environment, the capillary was immersed in 100 mM PBS solution (pH 7.5) containing 5.0% (w/v) BSA for 2 hours, and finally cleaned with 20 mM Tris-HCl (pH 7.5). For ATP and thrombin detection, one end of the capillary was


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dipped into a sample which contained the analytes and 1.0 µM corresponding aptamers labeled with FAM and Cy3, respectively. After the sample wicking into the capillary, the capillary was immersed in the sample and fully reacted for 30 min. The unreacted and non-specifically adsorbed reagents were rinsed using 20 mM Tris-Hcl (pH 7.5) for 3 times. Fluorescence microscope (BX53, Olympus) was used to obtain the fluorescent image of the capillary. For quantitative detection, a fluorescence microscope (IX71, Olympus) and an optical fiber spectrometer (QE 65000, Ocean optics) was used to detect the fluorescence signal. The position relationship between optical path and the sample is shown in Figure S1.



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RESULTS AND DISCUSSION The PCs reported here have multiple heterostructures that lead to fluorescent enhancements for different fluorophores. Therefore, it can be used for multiplex tests in a single channel. As shown in Figure 2a, the end of the capillary (500 µm in diameter) was immersed in the alcohol-based colloid solution containing monodisperse SiO2 NPs (195 nm in diameter). The colloid solution wicked through the capillary owing to capillary action. With solvent evaporation from the other end of the capillary, SiO2 NPs self assembles into PC in the capillary. By changing the colloid solution, SiO2 NPs with a diameter of 256 nm and 288 nm, respectively, were sequentially introduced into the capillary which lead to an opal capillary with three different photonic stop bands. Each portion of the capillary showed characteristic color (blue, green and red shown in Figure 2b), because light with corresponding wavelength is reflected by the periodic photonic structure in the capillary (Figure 2c and 2d). Besides, it is obvious that this fabrication method is compatible with parallel preparation of multiple capillaries at the same time (Figure 2e).


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Figure 2. (a) A photograph of the capillary inserted into colloid solution for the fabrication opal capillary (scale bar: 1 cm). (b) A photograph of the opal capillary prepared with three kinds of SiO2 NPs (195, 256 and 288 nm). The opal capillary exhibited different colors due to the reflection of the various PCs (scale bar: 0.25 cm). (c) Reflectance spectra of the PCs self-assembled using NPs with a diameter of 195, 256 and 288 nm, respectively. (d) Corresponding scanning electron micrographs of the PCs (scale bar: 0.50 µm). (e) Schematic illustration showing the parallel fabrication of multiple opal capillaries. PCs with heterostructure were also fabricated using this method. As shown in Figure 3, a glass capillary was used as the template. One layer of monodisperse SiO2 NPs (246 nm in diameter) was first assembled onto the inner wall of the capillary by introduction of a small amount of alcohol suspension of the NPs into the capillary (Figure 3a and 3b). After introduction of the colloid solution containing SiO2 NPs with a diameter of 228 nm, an opal capillary with PCs of heterostructure was fabricated (Figure 3c and 3d). The SEM image about the interface of the heterostructures is showed in Figure S2.



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Figure 3. Scanning electron micrographs of different capillaries. (a) Cross-sectional view of the capillary with small amount of SiO2 NPs (246 nm in diameter) assembled on the wall as the outer layer of the heterostructure. (b) Enlarged view of the area highlighted in (a) (scale bar: 2.5 µm). (c) Cross-sectional view of the capillary with heterostructures assembled using SiO2 NPs (228 nm, 246 nm and 274 nm in diameter). The outer layer was prepared using SiO2 NPs (246 nm in diameter). (scale bar: 100 µm). (d) Enlarged view of the area highlighted in (c) (scale bar: 10 µm). The periodic structures of the opal capillary lead to distinctive fingerprint in their reflection spectra. As shown in Figure 4, the opal capillary was fabricated by sequentially introducing three different colloid solutions containing SiO2 NPs with a diameter of 228, 246 and 274 nm, respectively. The optical photographs and the reflection spectra measured for these self-assembled colloid crystals were shown in Figure 4a. The reflection peaks were correlated to the diameters of the SiO2 NPs, which is governed by Bragg equation, λ =1.633×dnavg., where d is the center-to-center distance between neighboring particles, and navg. is the average refractive index of the PCs structure.23-24 For the opal capillary with the heterostructure that was comprised of two layers of self-assembled SiO2 NPs (228 and 246 nm in diameter), two reflection peaks were observed in the reflection spectra, as


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shown in Figure 4b. We found that the photonic structure of the inner layer lead to more obvious reflection peak compared to that of the outer layer, because the thickness of the inner layer was much larger than that of the outer layer. (Figure 3) Similar phenomenon has been observed for opal capillary with heterostructure fabricated using NPs with a diameter of 246 and 274 nm, respectively.

Figure 4. (a) Reflectance spectra of the opal capillary. The capillary was comprised of colloid crystals self-assembled using SiO2 NPs with a diameter of 228, 246 and 274 nm, respectively (scale bar: 500 µm). (b) Reflectance spectra of the opal capillary having the heterostructure. The heterostructure was composed of two layers (inner and outer) of self-assembled SiO2 NPs with a diameter of 228 nm and 246 nm, respectively. (c) Reflectance spectra of the opal capillary having two layers (inner and outer) of self-assembled SiO2 NPs with a diameter of 246 nm and 274 nm, respectively. The fluorescent enhancement of the prepared SiO2 colloid nanostructures for FAM and Cy3 was investigated. As shown in Figure 5a, the SiO2 colloid nanostructure showed considerable enhancement of the fluorescence after introduction of 2.0 µL of



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2.0 µM FAM into the capillary. Because the FAM has maximum fluorescence emission at 518 nm, which is overlapped with the peak reflection wavelength of nanostructure assembled using SiO2 NPs with a diameter of 246 nm (Figure 4a). So the fluorescent enhancement was larger than that for SiO2 NPs with a diameter of 228 nm (Figure 5a).

Figure 5. Fluorescent intensity measured from hollow capillary and opal capillary with different structures after introduction of 2.0 µL aliquot of 20 mM PBS (pH 7.5) containing 2.0 µM (a and b) FAM or (c and d) Cy3. (a and b) The opal capillary was fabricated using self-assembled SiO2 NPs with different diameters. (c and d) The opal capillary fabricated had two layers of SiO2 NPs sequentially self-assembled in the capillary. The diameters and the positions of the SiO2 NPs are indicated (scale bar: 500 µm). All error bars represent the standard deviation for three replicated tests. For the opal capillary with the heterostructure assembled using the SiO2 NPs with a diameter of 246 and 228 nm, respectively, the fluorescence from the same amount of FAM was further enhanced (by 33-fold compared with hollow capillary without nanostructure). This is because the double stopbands of the heterostructured opal


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capillary (485 and 525 nm) overlap with both the excitation and the emission wavelengths of FAM (492 and 518 nm) resulting in synergistic resonant enhancement.9, 25-26 We also found that the fluorescent enhancement effect of the heterostructure with 228 nm NPs in the outer layer was larger than that of nanostructures with 246 nm NPs in the outer layer. Cy3 is another fluorescent molecule widely used in all kinds of bioassays. Based on the investigation of fluorescent enhancement of FAM, we designed the heterostructured opal capillary for Cy3-based fluorescent assays. The fluorescence of Cy3 was enhanced by the opal structure assembled using 274 nm SiO2 NPs, because the maximum fluorescence emission wavelength of Cy3 (570 nm) overlaps with the peak reflection wavelength of the nanostructure (Figure 4a). With heterostructured capillary, the fluorescence of Cy3 was further enhanced by 40 fold owing to the synergistic resonant effect.

Figure 6. (a) An opal capillary for fluorescent detection of thrombin and ATP. The



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left half of the opal capillary was fabricated using self-assembled SiO2 NPs with a diameter of 228 nm (inner layer) and 246 nm (outer layer), respectively. The right half of the opal capillary was fabricated using self-assembled SiO2 NPs with a diameter of 274 nm (inner layer) and 246 nm (outer layer), respectively (scale bar: 0.5 cm). (b) Fluorescent signal as a function of thrombin concentration. (c) Fluorescent signal as a function of ATP concentration. All error bars represent the standard deviation for three replicated tests. To demonstrate the applicability of the opal capillary with heterostructures to multiplex bioanalysis, we fabricated a capillary for fluorescent detection of thrombin and ATP, as shown in Figure 6. The capillary was composed of two photonic heterostructures. The left half of the capillary was prepared using SiO2 NPs (246 nm in diameter) as the outer layer and SiO2 NPs (228 nm in diameter) as the inner layer. The right half of the capillary was prepared using SiO2 NPs (246 nm in diameter) as the outer layer and SiO2 NPs (274 nm in diameter) as the inner layer. By matching the excitation and emission wavelengths of FAM and Cy3 with the double stopbands of the photonic heterostructure, the fluorescence of both dyes were significantly enhanced. After introduction of sample and completion of the assays, the fluorescence from each half of the capillary was measured and correlated to concentration of ATP and thrombin, respectively. As shown in Figure 6b, a linear relationship between the fluorescent intensity and thrombin concentration was observed from 10 nM to 2000 nM. For ATP, the fluorescence intensity was linearly correlated to ATP concentration from 0.050 mM to 2.0 mM. The limit of detection (LOD), which was calculated as 3 times the standard deviation of the testing results of the blank divided by the slope of the calibration curve, is 8.1 nM for thrombin and is 32 µM for ATP, respectively. The LOD is lower than those of many complicated chemosensors with tediously operations for thrombin27-28 and ATP,

29

which demonstrates that photonic


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heterostructures could enhance the fluorescence signal thereby lower the detection limit significantly.



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SUMMARY AND CONCLUSIONS To summarize, we have reported a method inspired by plant transpiration for fabrication of opal capillary with multiple heterostructures. The heterostructures were prepared by self-assembly of SiO2 NPs in the capillary and were utilized for enhancing the fluorescence of FAM and Cy3. By matching the excitation and emission wavelengths of these fluorescent dyes with the stopband wavelengths of the photonic heterostructures, the fluorescence was amplified by up to 40 fold. The heterostructures were further modified by aptamers for multiplex bioassays. Thrombin and ATP, which are proof-of-concept analytes, were quantitatively detected with a LOD of 8.1 nM for thrombin and 32 µM for ATP, respectively. Therefore, this method is promising for fabrication of bioanalytical materials with complicated heterostructures for highly sensitive bioassays.


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ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI:

Schematic illustration showing the optical setup; A cross-sectional view of the capillary with heterostructures.

AUTHOR INFORMATION Corresponding Author

*To whom correspondence should be addressed. Email: [email protected], [email protected] Present Addresses

‡ Laboratory of Environment and Biosafety, Research Institute of Southeast University in Suzhou, Suzhou 215123, China # Getein Biotech, Inc. No.9 Bofu Road, Luhe Distric, Nanjing, Jiangsu, China (211505) Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. & These authors contributed equally to this work and should be considered co-first authors



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ACKNOWLEDGMENTS We gratefully acknowledge financial support from Chinese Recruitment Program of Global Experts, Innovative and Entrepreneurial Talent Recruitment Program of Jiangsu Province, the National Natural Science Foundation of China (21405014, 21327902, 21635001), the Natural Science Foundation of Jiangsu (BK20140619), the Science and Technology Development Program of Suzhou (ZXY201439), the Research Fund for the Doctoral Program of Higher Education of China (20120092130006),

State

Key

Project

of

Research

and

Development

(2016YFF0100802), the Project of Special Funds of Jiangsu Province for the Transformation of Scientific and Technological Achievements (BA2015067).


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ABBREVIATIONS

NPs, nanoparticles; ATP, adenosine triphosphate; PCs, photonic crystals; Cy3, cyanine 3 fluorescein; BSA, bovine serum albumin; Tris-HCl, Tris(hydroxymethyl) aminomethane; NPs, nanoparticles; FAM, 5-carboxyfluorescein.



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