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Flexible Antibacterial Film Based on Conjugated Polyelectrolyte/Silver Nanocomposites Xiaoyu Wang, Shuxian Zhu, Lu Liu, and Lidong Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00885 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 28, 2017

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ACS Applied Materials & Interfaces

Flexible Antibacterial Film Based on Conjugated Polyelectrolyte/Silver Nanocomposites Xiaoyu Wang,#,† Shuxian Zhu,#,† Lu Liu,† and Lidong Li*,†,‡ †

State Key Laboratory for Advanced Metals and Materials, School of Materials

Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China ‡

State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian

116024, P. R. China

ABSTRACT In this work, we report a flexible film based on conjugated polyelectrolyte/silver nanocomposites with efficient antibacterial activity. A flexible poly(dimethylsiloxane) film served as a substrate for deposition of nanostructured silver. A light-activated antibacterial agent, based on the cationic conjugated polyelectrolyte poly({9,9-bis[6′ -(N,N-trimethylamino)hexyl]-2,7-fluorenyleneethynylene}-alt-co-1,4-(2,5-dimethoxy) phenylene) dibromide (PFEMO) was self-assembled on the negatively charged substrate. By changing the thickness of the poly(L-lysine)/polyacrylic acid multilayers between the metal substrate and PFEMO, we obtained concomitant enhancement of PFEMO fluorescence, phosphorescence, and reactive oxygen species generation. These enhancements were induced by surface plasmon resonance effects of the Ag nanoparticles, which overlapped the PFEMO absorption band. Owing to the combination of enhanced bactericidal effects and good flexibility, these films have

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great potential for use as novel biomaterials for preventing bacterial infections. KEYWORDS:

self-assembly,

conjugated

polyelectrolyte,

fluorescence,

nanocomposite, antibacterial

INTRODUCTION Polymer substrates such as poly(dimethyl siloxane) (PDMS) are used in a broad range of biomedical applications like artificial skin and contact lenses owing to their biocompatibility and flexibility.1-3 However, these polymer substrates are susceptible to bacterial fouling during practical use.4 Attaching antibacterial agents to the surface of substrates is an important strategy to prevent bacterial infections.5-7 Thus, considerable efforts have been made to design and develop new antibacterial agents.8-10 In recent years, conjugated polyelectrolytes (CPEs) have aroused considerable interest as light-activated antibacterial agents.11,12 The conjugated polymer backbones of CPEs give strong light-absorption and high fluorescence quantum yields.13-18 A CPE can absorb light and transfer the energy to local oxygen molecules, producing reactive oxygen species (ROS) that can kill pathogens.19-21 When modified by positively charged pendant groups CPEs can be made water soluble, which enhances their interactions with negatively charged bacteria membranes.22,23 The degree of association between polymers and bacteria can modulate the light-activated antibacterial ability enabled by the ROS sensitization properties of the CPEs backbones. Compared with small organic molecule based antibacterial agents,

2


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polymer-based antibacterial agents have low residual toxicity, which reduces potential complications for environment and human health.24,25 Techniques commonly used to attach antibacterial agents to solid substrates include spin-coating, covalent grafting and layer-by-layer (LbL) self-assembly.26-28 The LbL approach can be used for CPEs with charged side chains to assemble layers on various substrates by electrostatic interactions.29,30 Films fabricated by LbL assembly, with careful selection of the adsorbed species, can have good biocompatibility, which may be favorable for biomedical applications.31-34 However, CPEs often suffer from self-aggregation on the film, which may decrease their fluorescence intensity. The metal-enhanced fluorescence (MEF) effect has been successfully used to modify the optical properties of CPEs.35 The phenomenon of MEF relates to the surface plasmon resonance (SPR) of metal nanoparticles. Excited plasmonic nanoparticles can modify the radiative properties of fluorophores and enhance the electromagnetic energy in the near-field.36-38 These two effects are respectively referred to as emission enhancement and excitation enhancement.39,40 As a result, a variety of optical processes, such as absorption, and fluorescence and phosphorescence emission, can be altered in the presence of excited plasmonic nanoparticles.41,42 We expect that preparation of CPE/metal nanoparticle composite films is a useful strategy for modifying and enhancing the light-activated antibacterial properties of CPEs. In

this

work,

we

prepared

a

flexible

film

based

on

conjugated

polyelectrolyte/silver nanocomposites for killing bacteria. The structure of the flexible 3



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film is illustrated in Scheme 1. The cationic conjugated polyelectrolyte poly({9,9-bis[6′-(N,N-trimethylamino)hexyl]-2,7-fluorenyleneethynylene}-alt-co-1,4(2,5-dimethoxy)phenylene) dibromide (PFEMO)43 was selected as a light-activated antibacterial agent to coat the surface. A PDMS film served as the flexible substrate for deposition of silver. Then, biocompatible poly(L-lysine) (PLL)/polyacrylic acid (PAA) multilayers, were used as a spacer between the metal substrate and PFEMO to control the MEF effects of the metal substrate on the PFEMO. The fluorescence intensity of PFEMO was enhanced on the flexible film. Along with the MEF effect, SPR-enhanced phosphorescence of PFEMO occurred, which increased ROS generation, indicating good potential for use of this flexible film as an antibacterial material.

Scheme 1. Schematic illustration of the flexible antibacterial film and chemical structures of PAA, PLL, and PFEMO. EXPERIMENTAL SECTION Materials and Measurements. Sylgard 184 was purchased from Dow Corning. Silver nitrate (AgNO3), ammonium hydroxide (NH3·H2O), glucose, and sodium 4


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hydroxide (NaOH) were purchased from Beijing Chemical Works. PAA (35 wt.% aqueous

solution,

Mw

=

100,000),

PLL

(Mw

=

30,000–70,000)

and

2′-7′-dichlorofluorescin diacetate (DCFH-DA) were purchased from Sigma–Aldrich. PFEMO was synthesized according to a published procedure.43 Phosphate buffered saline (PBS, pH 7.4) was purchased from HyClone. Escherichia coli (E. coli, TOP10) were purchased from Beijing Bio-Med Technology Development Co., Ltd. Ultrapure Millipore water (18.6 MΩ·cm) was used throughout the experiments. All reagents were used without further purification unless otherwise stated. Ultraviolet (UV)-ozone treatment was performed with a BZS250GF-TC UV/ozone

cleaning

system

(Shenzhen

H-Wo

Technology

Co.,

Ltd.).

Ultraviolet-visible (UV-vis) absorption spectra were recorded using a Hitachi U-3900H spectrophotometer. The surface morphology of PDMS@Ag was determined with a scanning electron microscope (SEM, JEOL JSM-7401F). Fluorescence spectra at room temperature and phosphorescence spectra at 77 K were obtained with a Hitachi F-7000 fluorescence spectrometer equipped with a Xenon lamp excitation source. The multilayer thickness was determined by a Bruker Dektak XT surface profiler. Photographs of the films were captured by a Nikon D-7000 camera. Time-domain lifetime measurements were measured by an Edinburgh Instruments F900 spectrometer with excitation at 450 nm. Transient absorption data were measured with an Edinburgh Instruments LP920 spectrometer, and argon-saturated ethanol was used in the measurements. A xenon lamp (CXE-350, Beijing OPT Photoelectric Technology Co., Ltd) was used as the white light source (400−800 nm). 5



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Preparation of Flexible PDMS@Ag Films. PDMS precursor Sylgard 184A was mixed with the crosslinking agent Sylgard 184B at a weight ratio of 10:1, and poured into a mold. The mixture was degassed under vacuum for 30 min and cured at 60 °C for 2 h, resulting in a PDMS film with a thickness of 1 mm. To form Ag nanoparticles on the surface of the PDMS substrate, first a silver ammonia solution (Ag(NH3)2OH) was prepared as follows: an aqueous solution of 25% NH3·H2O was slowly added into 15 mL of a 35 mg/mL aqueous solution of AgNO3 until the mixture became transparent. After a UV-ozone treatment, the PDMS substrate became hydrophilic44 and was then immersed in Ag(NH3)2OH solution to absorb [Ag(NH3)2]+ ions. After 30 min, the film was immersed in 90 mg/mL glucose solution at 60 °C with continuous stirring for 10, 30, 60 and 90 min, respectively. In this way, the PDMS@Ag films were prepared. The films were washed three times with deionized water and dried under a gentle stream of nitrogen before further use. Preparation of Flexible PDMS@Ag/(PLL/PAA)n/PFEMO Films. The prepared PDMS@Ag films were alternately immersed in solutions of oppositely charged PLL (3 mL, 0.5 mg/mL) and PAA (3 mL, 1.0 mg/mL) for 15 min. Following this method, multilayer films (PLL/PAA)n (n = 1–3) were built on the PDMS@Ag films. The outermost layer of PAA was allowed to adsorb the positively charged PFEMO. After immersing PDMS@Ag/(PLL/PAA)n in 5×10−4 M PFEMO solution for 15 min, a flexible PDMS@Ag/(PLL/PAA)n/PFEMO film was obtained. Control bare PDMS substrates were also prepared, without Ag nanoparticles, and covered with the same interlayers. 6


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Computational details. The optimized geometry of the ground state and the lowest-lying triplet state (T1) were optimized with density functional theory at the B3LYP and UB3LYP theory levels, respectively. There was no imaginary frequency for the optimized structures. All calculations were performed on the ORCA 3.0.2 quantum chemistry package45, with the same basis set Def2-SVP46. The RIJCOSX approximation was used to speed up the calculations with an auxiliary basis of Def2-SVP/JK for coulomb fitting and correlation fitting. To decrease the computation time, we replaced long alkyl side-chains with methyl groups in all calculations. ROS Detection of a Flexible PDMS@Ag/(PLL/PAA)2/PFEMO Film. The ROS probe 2ʹ,7ʹ-dichlorofluorescin (DCFH) was obtained by activation of DCFH-DA with NaOH solution.47 Typically, DCFH-DA solution (0.5 mL, 1 mM in ethanol) was added to the NaOH solution (2 mL, 10 mM) and maintained at room temperature for 30 min. The formed DCFH solution was then diluted with 10 mL of PBS and stored on ice in the dark before use. The final concentration of the DCFH solution was 40 µM. To detect ROS production, each film was immersed in 1 mL of DCFH solution, and the fluorescence spectra were measured at 1 min intervals while the film was irradiated with white light (90 mW/cm2). Antibacterial Assay on a Flexible PDMS@Ag/(PLL/PAA)2/PFEMO Film. A single colony of E.coli TOP10 on a solid Luria–Bertani (LB) agar plate was transferred to 10 mL of liquid LB culture medium containing ampicillin (50 µg/mL) and grown at 37 °C for 6 h. The bacteria were collected by centrifugation at 4,500 × g for 2 min and washed three times with PBS. The obtained bacteria were then 7



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re-suspended in sterile water and diluted to an optical density of 1.0 at 600 nm (OD600 =

1.0).

Next,

10

µL

of

the

bacterial

PDMS@Ag/(PLL/PAA)2/PFEMO,

solution

was

added

PDMS/(PLL/PAA)2/PFEMO

to

the and

PDMS@Ag/(PLL/PAA)2, substrates. One group was exposed to 90 mW/cm2 white light for 5 min, and another group was left in the dark for 5 min. The films were washed several times with 2 mL of PBS and the obtained bacteria solution was diluted 500 fold with PBS. A 100-µL portion of the diluted bacteria solution was spread on a solid LB agar plate containing ampicillin (50 µg/mL). After 12 h incubation at 37 °C, colonies formed and the number of colony-forming units (CFU) was counted. A 10-µL portion of the bacterial solution under the same light/dark conditions was used as a blank sample and then diluted 100,000 fold with PBS. A 100-µL portion of the diluted bacteria solution was spread on the same solid LB agar plate and incubated under the same conditions. The survival fraction was determined by dividing the number of CFU of the different samples by the number of CFU of the blank sample. RESULTS AND DISCUSSION Preparation and Characterization of a Flexible PDMS@Ag Film. First, the silver ion-loaded PDMS was immersed in glucose solution to prepare Ag nanoparticles and form the PDMS@Ag film. Then, an interlayer film composed of cationic polyelectrolyte PLL and anionic polyelectrolyte PAA was deposited via electrostatic attraction. Finally, positively charged PFEMO was deposited on the negative surface of the nanocomposite film to offer a bactericidal surface. Metal nanoparticle including silver and gold can be prepared with well

8


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controlled morphology and size.35,48,49 Nanostructured Ag is normally used in MEF study because its SPR absorption band is in the visible region and it shows a high scattering and enhancement efficiency.38 In this work, the Ag nanoparticles were selected as amplifying materials and prepared in-situ on the PDMS matrix. Silver ions adsorbed to the PDMS matrix and were then reduced by glucose to form Ag nanoparticles. As the silver ion-loaded PDMS was immersed into the glucose solution, the reduction reaction occurred immediately. The time evolution of the UV-vis absorption spectra of the films is shown in Figure 1a, illustrating the development of the Ag nanoparticles. The intensity of the Ag absorption band increased with reaction time, indicating a progressive reduction of silver ions. When the reaction time reached 90 min, the Ag nanoparticles exhibited a characteristic absorption peak at 408 nm, which was consistent with the surface plasmon resonance (SPR) absorption band of Ag nanoparticles. A slight blue-shift in the absorption band of the Ag nanoparticles may be attributed to an increase in the electron density of the particles.50 A visible color change of the films (from transparent to yellow) could be observed, further indicating that Ag nanoparticles were formed in the resulting PDMS@Ag film (Inset of Figure 1a). The surface morphology of the 90 min PDMS@Ag film was analyzed with an SEM. As shown in Figure 1b, the as-formed Ag nanoparticles had an average diameter of 35 nm and were randomly distributed throughout the PDMS matrix suggesting that efficient MEF effects could be obtained.29,35 The 90 min PDMS@Ag film was selected as the nanoparticle-modified substrate for further study. As shown in Figure 2, after loading the Ag nanoparticles, the elasticity and flexibility of the 9



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PDMS were well-preserved in the PDMS@Ag films.

Figure 1. (a) UV-vis absorption spectra of PDMS@Ag with different reduction times (10–90 min). Insert: Photographs of PDMS film and 90 min PDMS@Ag film, respectively. (b) SEM image of 90 min PDMS@Ag film.

Figure 2. Photographs showing bending of the PDMS@Ag film. Scale bar indicates 2 cm. MEF Effects of a Flexible PDMS@Ag/(PLL/PAA)n/PFEMO Film. To determine the effect of silver SPR on the emission properties of PFEMO, the fluorescence intensity of PFEMO from the surface of nanocomposite film was first studied. According to the principle of MEF effect, the extent of spectral overlap between SPR absorption band of the Ag nanoparticles and the absorption of fluorophores has an important influence on the fluorescence enhancement.51 Notably the absorption band of the PDMS@Ag film overlapped well with that of PFEMO (Figure 3a), which suggests the possibility of an interaction between the SPR of the 10


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Ag nanoparticles and the absorption of PFEMO. The MEF effect is a nanoscale phenomenon that becomes pronounced when a fluorophore is located at an optimal distance from the metal nanostructures.52 Thus, we produced a series of (PLL/PAA)n interlayers with different numbers of bilayers to vary the interlayer thickness and allow the distance between the Ag nanostructures and PFEMO to be optimized. The dependence of the PFEMO fluorescence intensity on the thickness of the (PLL/PAA)n interlayer is shown in Figure 3b. The fluorescence intensity of PFEMO increased to its maximum as the number of PLL/PAA bilayers was increased to two. The thickness of the (PLL/PAA)2 interlayer was determined to be 11 nm. However, the fluorescence intensity of PFEMO declined for the thicker (PLL/PAA)3 interlayer. Because the (PLL/PAA)2 interlayer gave the greatest enhancement of the PFEMO fluorescence, the

optimal

film

structure

for

our

system

was

set

as

PDMS@Ag/(PLL/PAA)2/PFEMO. The fluorescence intensity of PFEMO on the PDMS@Ag/(PLL/PAA)2/PFEMO structure was 2 times as high as that of the PDMS/(PLL/PAA)2/PFEMO (Figure 3c). To further investigate the enhancement in fluorescence emission, we measured the fluorescence

emission

lifetime

PDMS@Ag/(PLL/PAA)2/PFEMO

of

and

PDMS/(PLL/PAA)2/PFEMO. No notable increase of the radiative decay rate was observed for PFEMO in the presence of Ag nanoparticles (Figure S1). The calculated average lifetimes (Table S1) of PFEMO from PDMS@Ag/(PLL/PAA)2/PFEMO and PDMS/(PLL/PAA)2/PFEMO were 1.42 and 1.73 ns, respectively. This phenomenon suggests that the enhancement in fluorescence emission of PFEMO on 11



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PDMS@Ag/(PLL/PAA)2/PFEMO was mainly caused by excitation enhancement.39,53 As the excitation wavelength of the PFEMO was close to the SPR band of the Ag nanoparticles (Figure 3a), the excited plasmonic Ag nanoparticles increased the incident electromagnetic field and enhanced excitation of PFEMO. The strong emission

of

PFEMO

was

apparent

by

visual

inspection

of

the

PDMS@Ag/(PLL/PAA)2/PFEMO under UV light (Figure 3d). Figure 3d also shows that the PFEMO film was uniformly adsorbed to the surface of the nanocomposite film and that structure retained the elastomeric properties of the PDMS substrate.

Figure 3. (a) Normalized UV-vis absorption spectra of PDMS@Ag and PFEMO; normalized fluorescence emission spectrum of PFEMO in aqueous solution upon excitation at 407 nm. (b) The effect of interlayer thickness on the fluorescence intensity of PFEMO from PDMS@Ag/(PLL/PAA)n/PFEMO and the thickness of (PLL/PAA)n

interlayer,

n

=

1–3.

(c)

Fluorescence

12


emission

spectra

of

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PDMS/(PLL/PAA)2/PFEMO and PDMS@Ag/(PLL/PAA)2/PFEMO upon excitation at 407 nm. (d) Fluorescence images of PDMS@Ag/(PLL/PAA)2/PFEMO and the bended film following irradiation at 365 nm. Scale bar is 1 cm. ROS Production of a Flexible PDMS@Ag/(PLL/PAA)2/PFEMO Film. While MEF has been well documented, similar behavior can be expected for metal-enhanced phosphorescence and ROS production. A simplified Jablonski diagram is shown in Figure 4a. Enhanced excitation may enhance phosphorescence emission. A triplet excited photosensitizer can transfer its energy to oxygen to produce ROS.54 Thus, enhanced phosphorescence emission may result in an increase of ROS production and the triplet state of PFEMO plays a critical role in the ROS production. To investigate the excited triplet state of PFEMO, we performed quantum chemical methods on two repeating units of PFEMO using density functional theory.45,46 The T1 energy level of PFEMO was calculated to be 1.96 eV, which was high enough to produce ROS. Spin density plots of the T1 state, depicted in Figure 4b, suggested that the T1 excitons were distributed over a wide range of the molecule. This will likely increase the contact area between T1 excitons and oxygen and benefit ROS production. Furthermore, transient absorption spectra were measured to determine the triplet excited state of PFEMO. As shown in Figure S2, the transient absorption decay of PFEMO showed a triplet-triplet absorption maximum at 730 nm with a lifetime about 1.52 µs, indicating the presence of a triplet state.21 As the excited PFEMO can afford a triplet state that sensitizes oxygen molecules to produce ROS for killing bacteria, PFEMO is chosen for the antibacterial agent. The deposited PFEMO on the surface of the 13



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nanocomposite film could offer a bactericidal surface.

Figure 4. (a) Schematic energy level diagram for singlet oxygen generation by PFEMO. (b) Optimized geometry of the ground state and spin density plot of the T1 state of PFEMO. On the basis of the analysis of the triplet state of PFEMO, phosphorescence spectra were measured at 77 K. As shown in Figure 5a, a typical phosphorescence emission peak of PFEMO at approximately 690 nm was observed from both PDMS@Ag/(PLL/PAA)2/PFEMO

and

PDMS/(PLL/PAA)2/PFEMO.

The

Ag

nanoparticles induced a 1.9-fold enhancement of the PFEMO phosphorescence intensity for the PDMS@Ag/(PLL/PAA)2/PFEMO substrate, which was consistent with the phenomenon in the fluorescence emission. Production of ROS generation was detected with a ROS probe DCFH. ROS oxidize DCFH to fluorescent 2ʹ,7ʹ-dichlorofluorescein (DCF), which can be monitored to infer the presence of ROS.47 As shown in Figure 5b, upon continuous white light (400−800 nm) irradiation 14


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of DCFH in the presence of the different samples, the fluorescence intensity of DCF at 525 nm gradually increased for the samples containing PFEMO. After 10 min of irradiation, the emission increase of DCF for PDMS@Ag/(PLL/PAA)2/PFEMO was 2.9 times as high as that of PDMS/(PLL/PAA)2/PFEMO. This result clearly demonstrated the production of ROS from the PFEMO, and that greater ROS generation occurred from PDMS@Ag/(PLL/PAA)2/PFEMO. This increase could be attributed to metal-enhanced ROS production by the Ag nanoparticles.

Figure 5. (a) Phosphorescence emission spectra of PDMS/(PLL/PAA)2/PFEMO and PDMS@Ag/(PLL/PAA)2/PFEMO at 77 K upon excitation at 407 nm. (b) Fluorescence intensity of DCF at 525 nm under white light irradiation (1 mW/cm2) in the presence of PDMS@Ag/(PLL/PAA)2/PFEMO, PDMS/(PLL/PAA)2/PFEMO and PDMS@Ag/(PLL/PAA)2. The excitation wavelength was 488 nm. Antibacterial Assay on a Flexible PDMS@Ag/(PLL/PAA)2/PFEMO Film. We evaluated the antibacterial efficacy of the flexible films, having enhanced ROS production, in E. coli. Colony counting (Figure 6a) showed that the sample PDMS@Ag/(PLL/PAA)2 without PFEMO had almost no antibacterial activity under light

irradiation

or

in

the

dark.

The

bacterial

15


killing

efficiency

of


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PDMS/(PLL/PAA)2/PFEMO in the presence of PFEMO reached 41% when irradiated. However, PDMS@Ag/(PLL/PAA)2/PFEMO exhibited the highest killing efficiency, of 99%. Both PDMS/(PLL/PAA)2/PFEMO and PDMS@Ag/(PLL/PAA)2/PFEMO showed nearly 20% killing efficiency toward E. coli in the dark, which may be attributed to increased adsorption of E. coli to the positively charged PFEMO. These results demonstrated that the Ag nanoparticle-modified substrate affected the antibacterial activity

of PFEMO

on

PDMS@Ag/(PLL/PAA)2/PFEMO.

The

antibacterial effectiveness of PDMS@Ag/(PLL/PAA)2/PFEMO could be directly visualized

in

Figure

6b.

The

antibacterial

activity

of

the

PDMS@Ag/(PLL/PAA)2/PFEMO substrate toward E. coli upon light irradiation was particularly

pronounced.

Therefore,

the

optimized

flexible

PDMS@Ag/(PLL/PAA)2/PFEMO film exhibited the greatest potential for the photodynamic inactivation of bacteria.

16


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Figure

6.

(a)

Bactericidal

activity

of

PDMS@Ag/(PLL/PAA)2,

PDMS/(PLL/PAA)2/PFEMO and PDMS@Ag/(PLL/PAA)2/PFEMO toward E. coli in the dark and under white light irradiation, respectively. (b) CFU for E.coli incubated on PDMS@Ag/(PLL/PAA)2/PFEMO in the dark and under continuous white light irradiation (90 mW/cm2) for 5 min. CONCLUSION In this work, a flexible film based on conjugated polyelectrolyte/silver nanocomposites was prepared for killing bacteria. The formation of Ag nanoparticles in a biocompatible flexible PDMS film provided a nanostructured metal substrate. Cationic conjugated polyelectrolyte PFEMO could be self-assembled on the surface of the substrate as a light-activated antibacterial agent. The presence of biocompatible PAA/PLL multilayers between the metal substrate and PFEMO modified the MEF effects of Ag nanoparticles, leading to concomitant enhancement of PFEMO fluorescence, phosphorescence, and ROS generation. These enhancements resulted from excitation enhancement of the PFEMO through interaction with the SPR of the 17



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Ag nanoparticles. More importantly, SPR induced enhancement of ROS enhanced the antibacterial activity of the nanocomposite film, while retaining the elastomeric nature of PDMS. Therefore, our conjugated polyelectrolyte/silver nanocomposites films, with efficient antibacterial activity and good flexibility, are promising candidate materials for biocompatible antibacterial applications.

ASSOCIATED CONTENT Supporting Information Fluorescence

intensity

decay

PDMS@Ag/(PLL/PAA)2/PFEMO

of and

emission

at

488

PDMS/(PLL/PAA)2/PFEMO

nm at

of room

temperature. Exponential component analysis of the intensity decay of PFEMO measured by time-correlated single-photon counting. Transient absorption spectra of PFEMO in argon-saturated ethanol at different times after excitation. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (L.L.) Author Contributions #

X.W. and S.Z. contributed equally to this study.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS 18


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This work was supported by the National Natural Science Foundation of China (51373022, 51673022), the Fundamental Research Funds for the Central Universities (FRF-TP-16-026A1), the State Key Laboratory for Advanced Metals and Materials (2016Z-08) and the State Key Laboratory of Fine Chemicals (KF1613).

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