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A FACILE APPROACH FOR FABRICATION OF AG-ZN(OH)F
NETWORK-BASED MICROFLUIDIC DEVICE FOR SURFACE
ENHANCED RAMAN DETECTION
Z.Y. Zhu1,3†, G. Wang2,3†, Y. Guan1, and Y.F. Jin1,4*
National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Institute of
Microelectronics, Peking University, China, 2State Key Laboratory for Modification of Chemical
Fibers and Polymer Materials, College of Material Science and Engineering, Donghua
University, China, 3School of Chemical and Biomolecular Engineering, Georgia Institute of
Technology, USA, 4Peking University Shenzhen Graduate School, China
1
ABSTRACT
Ag-Zn(OH)F network-based microfluidic device is fabricated by flowing mixed AgNO3 and
Na3C6H5O7 solution into a glass capillary containing Zn(OH)F network. Surface-enhanced Raman
Scattering (SERS) measurements demonstrated that the microfluidic device is highly sensitive, stable,
reproducible, and recyclable.
KEYWORDS: Ag-Zn(OH)F, SERS, Microfluidic
INTRODUCTION
SERS is emerging as one of the most important techniques for providing the ultra-high-sensitivity
detection of organic pollutants with very low concentrations. High-aspect-ratio, nanostructured metallic
substrates have been proved to be promising SERS substrates. As the fundamental aspect of SERS
detection, the fabrication of substrate has attracted much attention in the past decades. Particularly, SERS
substrates containing Ag nanoparticles (NPs) are of special interest as Ag NPs can be synthesized from
inexpensive reagents with excellent SERS property. Although various chemical methods have been
developed as alternatives to prepare Ag NPs-based SERS substrate such as electrodeposition,1 electron
beam lithography,2 microcontact printing,3 and immersion coating,4 major efforts are still needed to
develop simple methods to fabricate cost-effective, flexible and portable Ag NPs-based SERS substrates.
Integrating SERS with microfluidic channel creates a promising platform for trace chemicals
detection with various advantages, including high-sensitivity, flexibility and portability, minimal use of
samples, and faster response time. Moreover, developing recyclable SERS substrates is significant
regarding cost reduction. Therefore, it is important to develop a simple route for fabricating a flexible,
portable and recyclable SERS device by taking advantages of both Ag NPs and microfluidic channels.
In this paper, we demonstrate a facile route for fabricating Ag NPs-based device by flowing the
mixed solution made of AgNO3 and Na3C6H5O7 into a Zn(OH)F network in a capillary at 80ºC for 20
min. The successful synthesis of Ag NPs was confirmed by scanning electron microscopy (SEM),
energy dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM). The device
shows high sensitivity, recyclability which emanates from photocatalytic self-cleaning, and
reproducibility.
EXPERIMENTAL
Silica glass capillaries with an internal diameter of 530 μm and a polyimide outer coating were used.
Chemical solutions to be reacted were drawn into two syringes which were attached to a dual-syringe
infusion pump and connected by two pinheads to a prefabricated ‘Y’-pattern Teflon tubes. The ‘Y’pattern Teflon tubes were connected to the capillary microchannel with epoxy resin. Then, the
microchannel was filled with chemical solution as the pump drove solutions through the Teflon tubes.
The Zn(OH)F networks were fabricated inside the silica glass capillaries in a two-step process
described in literature5 with slight modifications. First, ZnO were seeded on the inner walls of capillary.
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19th International Conference on Miniaturized
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October 25-29, 2015, Gyeongju, KOREA
Solutions of Zn(O2CCH3)2 (0.01 M) in C2H6O and NaOH (0.04 M) in C2H6O were driven into the ‘Y’shape tubes at 100 μL/min. When the microchannel was fully blocked by grown ZnO structures, the
pump was stopped and the microchannel was placed in an oven at 60ºC for 4 h. The microchannel was
then annealed at 150 ºC to enhance the adhesion of ZnO nanocrystals to microchannel. Second, Zn(OH)F
networks were prepared at 90ºC by replacing the two previous solutions with aqueous solutions of
Zn(NO3)2 (0.05 M) and NH4F (0.05 M) in the capillary microchannel over 1.5 h at 60 mL/min. After the
reaction was complete, the capillary microchannel was washed with distilled water and dried.
An AgNO3 solution (4 mL, 6 mM) was added to Na3C6H5O7 solution (0.09 ml, 60 mM). The mixed
solution was pumped from two syringes into a capillary microchannel containing Zn(OH)F networks at a
rate of 50 μL/min for 20 min. The capillary was placed in an oven at 80℃ during the synthesis process.
RESULTS AND DISCUSSION
The typical SEM image of the fabricated structures on the inner surface of the microchannels is
shown in Figure 1(a). The nanorods interlaced with one another and formed a network-like structure. The
EDX data of the fabricated network in Figure 1(b) shows the presence of O, F, Zn, Si, Au and Ag signals.
The Au signal was detected due to the sputtered Au layer on samples to enhance the SEM quality. The
EDX data indicated that Ag nanoparticles were successfully assembled to the Zn(OH)F network. A
typical TEM image of the fabricated sample is shown in Figure 1(c), exhibiting that nanorods were
covered with dark dots which were ascribed to Ag NPs. An enlarged HRTEM image in Figure 2(d)
showed that the interplanar spacing of 0.23 nm corresponded to lattice spacing for the (111) face of cubic
Ag. Together, the above results suggested the successful fabrication of Ag-Zn(OH)F network.
Figure 1: (a) Typical SEM image; (b) EDX spectra; (c) TEM image; and (d) HRTEM image of the
resulting structure on the inner surface of the microchannel.
The photocatalytic property of fabricated Ag-Zn(OH)F network is the premise for recyclable SERS
transducer. The Ag-Zn(OH)F network can clean itself by pohotcatalytic degradation of the organic
pollutants absorbed on the surface, thus the SERS transducer is recyclable. Figure 2(a) shows the photodegradation performance of Rhodamine 6G (R6G) with Ag-Zn(OH)F network. R6G can be degraded by
95-97% in 30 s. Figure 2(b) illustrates the proposed mechanism for the photocatalytic process.
Photoelectron transfer from Zn(OH)F to Ag occurs upon UV excitation of Zn(OH)F. The transferred
electrons at the surface of Ag then produce ˙O2- radicals, while the holes from Zn(OH)F form ˙OH
radicals. The R6G molecules react with these radicals and degrade into small molecules.
(a)
(b)
e e
UV
O2
H2O
Ag e
h+
˙O2-
˙OH
Zn(OH)F
Figure 2: (a) photo-degradation performance of R6G with Ag-Zn(OH)F network; (b) Scheme of the
photocatalytic mechanism.
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Concentrations of R6G in aqueous solution in the range of 10-9 M-10-13 M were used for Raman
analysis. Figure 3(a) shows that the detection limit was extended to 10-13 M. Further, mixtures of R6G
and 4-methyl benzoic acid (4-MBA) were prepared to investigate the detection capability for
multicomponent systems. Figure 3(b) shows SERS spectra of individual probe, and a mixture of the two
components, respectively. Each substance is readily discerned in the mixed solution.
For low cost applications, the ability to reuse the substrate is attractive. Thus, the effectiveness of AgZn(OH)F network in analyte detection after recycling was explored. After the initial SERS measurement,
the microchannel was cleaned with deionized water and simultaneously subjected to UV irradiation for 1
min. The samples were then rinsed with water and dried. In total, the “detection/self-cleaning” cycle was
performed 10 times. Figure 3(c) depicts R6G Raman spectra obtained after the 1st, 5th and 10th cycle.
(a)
(b)
(c)
Detection
I
UV-cleaning
Intensity (a.u.)
II
III
IV
V
10th
R ama
n shif
5th
t (cm -1
)
1600
1st
Figure 3: (a) SERS spectra of R6G at concentrations of I) 10-9 M, II) 10-10 M, III) 10-12 M, IV) 10-13 M, V)
0 M using the microfluidic device; (b) SERS spectra obtained from R6G (10-10 M) at 1st, 5th and 10th
cycles using the microfluidic device; c) SERS spectra of R6G, 4-MBA, and a mixture of the two analytes.
CONCLUSION
A new and facile route for fabricating a recyclable microfluidic SERS sensor comprising a 3D AgZn(OH)F network within a microchannel is demonstrated. Raman detection of representative analytes
confirmed that the 3D network can be used as a high sensitivity, stable, and recyclable SERS-active
substrate. This sensor has promising applications in flexible, portable and wearable microdevice.
REFERENCES
[1] Y. X. Huang, L. Sun, K. P. Xie, Y. K. Lai, B. J. Liu, B. Ren and C. J. Lin, “SERS study of Ag nanoparticles electrodeposited on patterned TiO2 nanotube films,”J. Raman Spectrosc.,42, 986-991,
2010.
[2] N. Felidj, J. Aubard, G. Levi, J. Krenn, A. Hohenau, G. Schider, A. Leitner and F. Aussenegg, “Optimized surface-enhanced Raman scattering on gold nanoparticle arrays,” Appl. Phys. Lett. 82, 3095,
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[3] V. Santhanam and R. P. Andres, “Microcontact Printing of Uniform Nanoparticle Arrays,” Nano
Lett. 4, 41-44, 2004.
[4] H. J. Yin, Y. F. Chan, Z. L. Wu, and H. J. Xu,“Si/ZnO nanocomb arrays decorated with Ag nanoparticles for highly efficient surface-enhanced Raman scattering,” Opt. Lett. 39, 4184-4187, 2014.
[5] G. Wang, G. Y. Shi, H. Z. Wang, Q. H. Zhang and Y. G. Li, “In Situ Functionalization of Stable 3D
Nest-Like Networks in Confined Channels for Microfluidic Enrichment and Detection,” Adv. Funct.
Mater. 24, 1017-1026, 2014.
AUTHOR CONTRIBUTIONS
† These two authors contributed equally to this work.
CONTACT
*Yufeng Jin; Tel: +86-10-62752536; [email protected]
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