Letter Cite This: ACS Macro Lett. 2019, 8, 223−227
pubs.acs.org/macroletters
In Situ Monitoring of Fluorescent Polymer Brushes by AngleScanning Based Surface Plasmon Coupled Emission Yu-Hua Weng,† Lin-Tao Xu,† Min Chen,† Yan-Yun Zhai,† Yan Zhao,† Shyamal Kr Ghorai,† Xiao-Hui Pan,† Shuo-Hui Cao,*,†,‡,§ and Yao-Qun Li*,† †
Department of Chemistry and the MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People’s Republic of China ‡ Department of Electronic Science, Xiamen University, Xiamen 361005, People’s Republic of China § Shenzhen Research Institute of Xiamen University, Shenzhen 518000, People’s Republic of China ACS Macro Lett. Downloaded from pubs.acs.org by TULANE UNIV on 02/08/19. For personal use only.
S Supporting Information *
ABSTRACT: Fluorescent polymers have attracted interest in many fields such as sensing, diagnostics, imaging, and organic electronic devices. Real-time techniques to monitor and understand the polymerization process are important for obtaining controllable fluorescence polymers. We present a new technique to in situ monitor the growth process of fluorescent polymer brushes by using angle-scanning based surface plasmon coupled emission (AS-SPCE) approach during electrochemically mediated atom-transfer radical polymerization. The polymer thickness was determined by modeling the location of SPCE emission angle(s) with theoretical calculation. The advantages of unique angle distribution patterns, thickness dependence and effective background rejection of AS-SPCE guarantee the success in the real-time investigation for controllable fabrication of fluorescent polymers. luorescent polymers have attracted interest in many fields, such as sensing, diagnostics, imaging, and organic electronic devices.1−6 Fluorescent polymers can be fabricated through postmodification of polymer chains with a dye7−9 and direct (co)polymerization of fluorescent dyes by ATRP (atom transfer radical polymerization),10−13 NMP (nitroxide-mediated polymerization),14,15 RAFT (reversible addition−fragmentation chain transfer),16,17 or LAP (living anionic polymerization).10,18 The direct (co)polymerization of fluorescent monomers takes the superiority of less time-consuming and lower cost compared with the postmodification method. Realtime techniques to monitor and understand the polymerization process are of vital importance and are urgently required to be developed.19−25 Because of high sensitivity and noninvasiveness, fluorescent-based techniques have been applied for an in situ study of polymerization.26−29 However, most fluorescentbased approaches gain merely insight on the degree of conversion of monomers to polymers rather than the polymers growth process, except that Tang et al. recently reported the molecular weight of polymers being in linear relationship with the photoluminescence intensity of the polymers doped with aggregation-induced emission luminogens.30 Surface plasmon coupled emission (SPCE), based on the near-field interaction between fluorophores and surface plasmons on metal surfaces, displays unique optical properties including strongly directionality, unique polarization, wavelength resolution, thickness dependence, and background suppression.31−34 Concerning thickness dependence, the incident angle(s), the emission angle(s), and the polarization
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© XXXX American Chemical Society
are strongly dependent on the thickness of the dielectric medium on the metal surface.35−44 The properties of thickness dependence and the background suppression enable SPCE to be a potential tool to investigate the polymerization process. In this report, an approach basing on angle-scanning surface plasmon coupled emission (AS-SPCE) is proposed as an efficient platform to in situ monitor a polymer growth process. Electrochemically mediated atom-transfer radical polymerization (eATRP)45−51 is an effective and controllable way to prepare polymers in the presence of ambient air and was first used to verify the feasibility of present method. Convenient recording of emission angular distribution of SPCE for distinguishing polymer thickness at various excitation angles was achieved, interpreting the growth process through fluorescence specifically originating from the bound fluorophores on the polymers, meanwhile avoiding the interference from mass of unbounded fluorophores in the solution. As shown in Figure 1, a homemade setup of AS-SPCE/ eATRP was built up to modulate the synthesis and to in situ measure the polymerization process. The angle-scanning section of AS-SPCE was designed through precisely and automatically controlling a coaxial rotary stage on which samples were positioned. A Kretschmann (KR) configuration was applied in this study, with an excitation laser and a Received: November 14, 2018 Accepted: February 4, 2019
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DOI: 10.1021/acsmacrolett.8b00882 ACS Macro Lett. 2019, 8, 223−227
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ACS Macro Letters
Figure 1. Schematics of AS-SPCE/eATRP setup with KR excitation for monitoring of fluorescent polymer brushes. Bottom schematically shows the growth of RhB-PAAm polymer brushes through eATRP.
detector separately scanning around the prism.31,35,43 In ASSPCE with KR excitation, through the prism coupler, only at those defined angles could the excitation field be efficiently coupled to the plasmonic substrate for selective illuminating and exciting the bound fluorophores on the polymer brushes, thus naturally avoiding the interference coming from the photoluminescence of abundant unbounded fluorophores in the bulk solution (Figure S1a, Supporting Information). On the contrary, if the light was incident from the front of the solution, intense fluorescence, generated from the bulk fluorophores (Figure S1b, Supporting Information), would greatly overwhelm the signal from the bound fluorophores. To be more important, the thickness of the fluorescent polymer brushes can be sensitively characterized by the unique emission angle distribution pattern in SPCE with high directionality and polarization, which could be regarded as the new indicator to reflect the growing situation of polymers. The eATRP reaction was carried out in a one-compartment, three-electrode electrochemical cell filled with a solution containing acrylamide (AAm) monomer, rhodamine B-acrylamide (RhB-AAm) fluorescent monomer, and Cu(II)/bipy. A 50 nm Au thin film modified with a mixed monolayer of initiator (SH-C15Br/2-NAT in a 1:9 ratio) was, on the one hand, used as the working electrode to realize eATRP and, on the other hand, used as the plasmonic substrate to couple fluorescence signal from the attached fluorescent polymers. A platinum plate and a Ag/AgCl/Cl− electrode acted as the counter electrode and the reference electrode, respectively. A constant potential of −0.6 V (Figure S2, Supporting Information) was applied to generate the Cu(I)/bipy complex through one-electron reduction of Cu(II)/bipy, thus, efficiently initiating the copolymerization in the vicinity of initiator-decorated gold surface. Figure 2 shows SPCE emission angular scan curves recorded during the eATRP reaction. Prior to the polymerization, very weak fluorescence was observed, which was generated from
Figure 2. SPCE emission angular scan curves recorded for 580 nm light after 0 min (a), 50 min (b), and 150 min (c) polymerization: ppolarized emission (red line), s-polarized emission (blue line). Insets show schematics of AS-SPCE setup for monitoring of polymer brushes. The vertical of the gold substrate was defined as 90°. The incident angles (θincident) used for excitation were marked in the figure.
free fluorophores near the metal film, since no fluorescent polymer was attached on the gold film (Figure 2a). After 50 min, a strong s-polarized fluorescence signal sharply concentrated at 61° was observed, but no p-polarized fluorescence was observed with KR excitation at 61° (Figure 2b), implying that the fluorescent polymer brushes had been formed on the gold film through eATRP. The measured angle perfectly coincided with the angle of minimum reflectivity calculated using Fresnel calculation for the 220 nm polymer (Figure S3d, Supporting Information). Increasing the polymerization time up to 150 min, the s-polarized fluorescence concentrated at 61° and 68° and the p-polarized fluorescence at 66° were 224
DOI: 10.1021/acsmacrolett.8b00882 ACS Macro Lett. 2019, 8, 223−227
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ACS Macro Letters observed with a KR excitation at 61° (Figure 2c), indicating the increase of the polymer thickness. The different angle distribution patterns resulted from the change in the reflective index with the increased polymer thickness, which could influence the coupling between fluorophores and plasmons. The alternative s- and p-polarized emission angles of SPCE were coincident with the calculated angles of minimum reflectivity for the 740 nm polymer (Figure S3f, Supporting Information). The results indicate that the angle distribution patterns could be efficient parameters to trace the thickness of polymers. SPCE emission angles were measured and plotted as a function of the polymerization time (Figure 3a). The polymer
Figure 4. Fluorescence microscopy images of the bare gold substrate (a) and the RhB-PAAm polymer brushes by eATRP (b). AFM images of the bare gold substrate (c) and the RhB-PAAm polymer brushes by eATRP (d).
In summary, we have demonstrated an accessible approach of AS-SPCE to in situ monitor the polymerization process for fabricating controllable fluorescent polymers. Attributed to ASSPCE with the capacity to determine the fluorescence angle distribution in a flexible and precise way, the characteristic thickness-dependent angle distribution patterns with unique polarization have been successfully used for investigating the thickness growth of fluorescent polymer brushes. AS-SPCE can be extended to trace the construction of various fluorescent sensors, as well as being a platform for sensing and imaging.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00882. Detailed information on chemicals, instrument, metallic substrate preparation and parameters for the calculation of the reflectivity; figures of SPCE emission angular scan curves recorded in the polymerization solution with KR and RK, cyclic voltammograms of the polymerization solution, reflectivity curves calculated of RhB-PAAm polymers for 532 and 580 nm incident light, and real photos of the bare gold film and the RhB-PAAm polymer by eATRP (PDF).
Figure 3. (a) Dependence of the SPCE emission angles observed on polymerization time. Black dots indicate the s-polarization and the red dots indicate the p-polarization. Inset shows the dependence of the optimal incident angle on the polymerization time. (b) Dependence of the polymer thickness determined by modeling data from (a) with Fresnel calculation on polymerization time. The error bars represent the standard deviations of three independent measurements.
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thickness was determined by modeling the location of the SPCE emission angle(s) with theoretical calculation on the polymerization time. Figure 3b shows the dependence of the polymer thickness on the polymerization time. A linear increase in the thickness of polymer from 0 to 800 nm within 150 min demonstrated the capability to realize a controllable polymer fabrication with the aid of in situ AS-SPCE monitoring. The strong fluorescence under fluorescence microscopy further demonstrated the successful formation of fluorescent polymer on the gold film (Figure 4b), though the red polymer film can be observed by naked eyes (Figure S4b, Supporting Information). The morphology of the polymer brushes was probed by AFM to show a smooth and homogeneous film (Figure 4d).
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Yao-Qun Li: 0000-0001-7842-1671 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge supports from National Natural Science Foundation of China (Grant Nos. 21874110, 21375111, 21505109, 21521004), the 973 Program of China (2013CB933703), the funds of the Ministry of Education of 225
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