Truxene-Based Hyperbranched Conjugated ... - ACS Publications

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Article

Truxene-Based Hyperbranched Conjugated Polymers; Fluorescent Micelles Detect Explosives in Water Wei Huang, Emanuel Smarsly, Jinsong Han, Markus Bender, Kai Seehafer, Irene Wacker, Rasmus R. Schroeder, and Uwe H. F. Bunz ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12419 • Publication Date (Web): 04 Jan 2017 Downloaded from http://pubs.acs.org on January 12, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Truxene-Based Hyperbranched Conjugated Polymers; Fluorescent Micelles Detect Explosives in Water Wei Huang,† Emanuel Smarsly,† Jinsong Han,† Markus Bender,† Kai Seehafer,† Irene Wacker, § Rasmus R. Schröder§ and Uwe H. F. Bunz*,†,‡

†

Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im

Neuenheimer Feld 270, 69120 Heidelberg, Germany

‡

CAM, Centre for Advanced Materials, Ruprecht-Karls-Universität Heidelberg, Im

Neuenheimer Feld 225, 69120 Heidelberg, Germany

§

Cell Networks, Bioquant, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer

Feld 225, 69120 Heidelberg, Germany

KEYWORDS: truxene, conjugated polymers, micelles, sensor, nitroarene detection

ABSTRACT: We report two hyperbranched conjugated polymers (HCP) with truxene units as core and 1,4-didodecyl-2,5-diethynylbenzene as well as 1,4-bis(dodecyloxy)-2,5-diethynylbenzene

as

co-monomers.

Two

analogous

poly(para-phenyleneethynylene)s (PPE) are also prepared as comparison to demonstrate the difference between the truxene and the phenyl moieties in their

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 24

optical properties and their sensing performance. The four polymers are tested for nitroaromatic analytes and display different fluorescence quenching responses. The quenching efficiencies are dependent upon the spectral overlap between the absorbance of the analyte and the emission of the fluorescent polymer. Optical fingerprints are obtained, based on the unique response patterns of the analytes towards the polymers. With this small sensor array, one can distinguish nine nitroaromatic analytes with 100% accuracy. The amphiphilic polymer F127 (a polyethyleneglycol-polypropyleneglycol block copolymer) carries the hydrophobic HCPs and self-assembles into micelles in water, forming highly fluorescent HCP micelles. The micelle bound conjugated polymers detect nitroaromatic analytes effectively in water and show an increased sensitivity compared to the sensing of nitroaromatics in organic solvents. The nitroarenes are also discriminated in water using this four-element chemical tongue.

1. INTRODUCTION

Sensing of nitro-group carrying arenes is an important task, as these species are not only degradation products of explosives contained in land mines, but also recognized as pollutants.1 Nitroaromatics pollution of soil and water is suspected to cause neuronal and internal organ damage.2,3 Typical representatives are nitrobenzene, dinitrobenzene, nitrotoluenes, dinitrotoluenes, trinitrotoluene and trinitrophenol (picric acid). Therefore, sensing of nitroaromatics in aqueous solution is an attractive 2 ACS Paragon Plus Environment

Page 3 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and important issue. Strategies for explosives detection include trained canines,4 analytical techniques (X-ray diffraction, mass spectrometry, cyclic voltammetry, nuclear quadrupole resonance and optical based detection).5-7 Among the available methods for explosives detection, optical sensing, reporting the change of fluorescence readout of poly(para-pheneyleneethynylene) (PPE)-type conjugated polymers, first introduced by Swager, is attractive due to its high sensitivity, quick response time and portability, compared with other methods (expensive, complicated and unsuitable).8-11 Sensors based upon fluorescence modulation include conjugated organic

molecules,

nanoparticles,

metal

complexes

and

metal-organic

frameworks.4,12-14 Conjugated polymers are superb sensor candidates owing to Swager’s “molecular wire” effect, whereby the signal of the analyte is 50-100-fold increased, when compared to that of monomeric sensors.15,16 Different fluorescent conjugated

polymers

are

reported

for

explosive

detection,

such

as

poly(tetraphenylethene),17 poly(para-phenyleneethynylene)s18 and polyacetylenes.19 However, suitable fluorescent conjugated polymers with high sensitivity and selectivity are still in demand, particularly if they are applied in aqueous condition. Electron-donating moieties effectively enhance the interaction between the electron-rich polymers and the electron-deficient explosives, and result in higher sensitivities and selectivities.20,21 The star-shaped truxene, with its unique trigonal topology is an attractive building motif for constructing electron-rich, extended π-conjugated polymers.22 By taking advantage of the structural and photophysical 3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 24

properties, we synthesized two truxene-based polymers. To our knowledge, truxene based polymers have not been reported as chemosensors for the detection of explosives. We also synthesized two analogous poly(para-phenylene-ethynylene)s as comparison to demonstrate the difference between truxene and benzene moieties in the polymers, and their sensing effect. Herein, we report four polymers which could detect explosive analytes and successfully discriminate explosives. Pluronic F-127, a triblock copolymer of poly(oxyethylene)-block-poly(oxypropylene)-block- poly(oxyethylene) is a commercial, non-toxic, low cost surfactant; F-127 carries hydrophobic polymers into the aqueous phase for applications in drug delivery systems.23,24 F-127 self-assembles into nanomicelles in aqueous solution, which solubilize hydrophobic polymers. It carries HCPs as sensing cores into water where we can detect nitroaromatic species; leaking from explosives buried in bodies of waters, ticking environmental time bombs. While Swager’s PPE-based systems are very successful in detecting nitroaromatics and therefore explosives in the gas phase, detection of explosives in water is less explored.

2. RESULTS AND DISCUSSION Synthesis and Characterization. Two hyperbranched conjugated polymers (HCP-1 and HCP-2) were synthesized by Sonogashira coupling of truxene modules and phenyleneethynylene struts, carrying alkyl and alkyloxy chains (Figure 1).25 For comparison two analogous 4 ACS Paragon Plus Environment

Page 5 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


poly(para-phenyleneethynylene)s

(PPEs)

with

identical

phenyleneethynylene

backbones were prepared. Structures and habitus (under daylight and UV light) of the four polymers are shown in Figure 1. The number-average molecular weights (Mn) of these polymers are estimated by SEC and their polydispersity (Đ) is reported with values of 1.2*104, 1.2*104, 1.5*105 and 4.8*104 g/mol (HCP-1, HCP-2, PPE-1 and PPE-2). The molecular weights of the HCPs are lower than those of the PPEs, because high degree of polymerization will result in insoluble HCPs, useless for further investigations. Đ (Mw/Mn) for HCP-1, HCP-2, PPE-1 and PPE-2 are 1.5, 1.9, 3.0, and 1.7 ( Table 1) respectively.

Figure 1. Structures and habitus (left: daylight, right: UV light) of HCP-1, HCP-2, PPE-1 and PPE-2.


HCP-1 Abs HCP-1 Em 1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 400

500

600

1.0

HCP-2 Abs HCP-2 Em 1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0

700

300

400

Wavelength(nm)

PPE-1 Abs PPE-1 Em 1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 300

0.0 400

500

600

(d) Normalized Absorbance

1.0

Normalized Emission

(c)

500

600

700

Wavelength(nm)

1.0

PPE-2 Abs PPE-2 Em 1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 300

Wavelength (nm)

Normalized Emission

300

(b) Normalized Absorbance

1.0

Normalized Absorbance


(a)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 24

Normalized Emission


0.0 400

500

600

Wavelength (nm)

Figure 2. Normalized absorption and emission spectra of (a) HCP-1, (b) HCP-2, (c) PPE-1 and (d) PPE-2 in THF.

The photophysical properties of the polymers (THF) are shown in Figure 2 and Table 1. In the fluorescence spectra, HCPs (402 nm, 428 nm) with truxene units absorb and emit at shorter wavelengths than the high molecular weight PPEs (424 nm, 471 nm). The maximum emission peaks of polymers (HCP-1 and PPE-1) with alkyl chains display a blue shift compared with polymers containing alkyloxy chains (HCP-2 and PPE-2). The fluorescence quantum yields (ΦF) for HCP-1, HCP-2, PPE-1 and PPE-2 are 56%, 65%, 80% and 80% respectively in THF. PPE-1 and PPE-2 display a higher fluorescence quantum yield than HCP-1 and HCP-2.


Page 7 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Photophysical Properties and GPC Data for HCP-1, HCP-2, PPE-1 and PPE-2 a

a

a

b

polymer

λmax,abs [nm]

λmax,em [nm]

ΦF [%]

Mn [g/mol]

Mw/Mn

HCP-1

320

402

56.1

11821

1.5

HCP-2

320

428

64.5

11878

1.9

PPE-1

406

424

80.2

149920

3.1

PPE-2

445

471

79.8

47966

1.7

a

b

Determined in THF. bDetermined by GPC in THF.

The four polymers detect nitroaromatics in solution via quenching. Nine different analytes (Figure 3a) were investigated, including seven nitroaromatics: picric acid (PA),

nitrobenzene

(NB),

dinitrobenzene

(DNB),

dinitrotoluenes

(DNT),

trinitrotoluenes (TNT), nitrophenol (NP), 2-nitroaniline (2-NA), 3-nitroaniline (3-NA) and aniline (A) as a comparison. The fluorescence intensity was gradually quenched after adding the analyte into the solution. To quantify the quenching efficiencies of the polymers to the analytes, we calculated the Stern-Volmer constants (KSV) according to the standard Stern-Volmer equation; however, the linear behavior was poor. Better fitting plots are achieved by a modified Stern-Volmer equation (1).26,27 KSV is the Stern-Volmer constant, I0 is the initial fluorescence intensity of the fluorophore (HCPs and PPEs), Ifinal is the final fluorescence intensity of the fluorophore, Iq is the fluorescence intensity at a given quencher (nine analytes) concentration, [F] is concentration of the fluorophore and [Q] is the total 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 24

concentration of the added quencher Q. The non-linear nature of the Stern-Volmer plots suggest a combination of static and dynamic quenching or an energy transfer process between the polymers and analytes.4,28

‫ܫ‬௤ = ‫ܫ‬଴ +

ூ೑೔೙ೌ೗ିூబ ଶ

ሾொሿ

× ቊ1 + ሾிሿ +

ଵ

௄ೞೡ

ሾொሿ

− ൤ቀ1 + ሾிሿ + ሾிሿ

ଵ

௄ೞೡ

ଶ

ሾொሿ

ଵ/ଶ

ቁ − 4 ሾிሿ൨ ሾிሿ

ቋ (1)

(a )

(b)

16000 12000

8000

Ksv 4000

A NB 3-NA DNT DNB TNT NP PA 2-NA

HCP-1 HCP-2 PPE-1 PPE-2

Figure 3. (a) Structures of tested nitro analytes. (b) Fluorescence quenching efficiencies of HCP-1, HCP-2, PPE-1 and PPE-2 for different analytes in THF. The z-axis denotes the Stern-Volmer constant Ksv.

The fluorescence quenching efficiencies of the polymers and the KSV values for nitroanalytes were in the order of 2-NA > PA > NP > TNT > DNB > DNT > 3-NA > NB > A (Figure 3, Table S1). When comparing 2-NA, NB and A, one notices that A or NB do not show strong quenching effects. However, 2-NA results in the highest 8 ACS Paragon Plus Environment

Page 9 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fluorescence quenching to our polymers. When it comes to the polymers, the four polymers give a trend of quenching constants: HCP-1 has the highest quenching efficiency, followed by HCP-2, PPE-1 and PPE-2 (HCP-1 > HCP-2 > PPE-1 > PPE-2). Although PPE-1 is more sensitive towards 2-NA, than HCP-2, HCPs are better sensor elements in most cases. We have also investigated the KSV values of HCP-1 for the detection of analytes (DNT, NP, PA and 2-NA) in THF, CHCl3 and toluene. The values are quite close for the same analyte in the three solvents; the KSV values are independent from the solvents (Figure S1). Figure 4 shows the normalized absorbance spectra of the analytes and the emission spectra of the polymers (300-600 nm, full range spectra see Figure S2). The quenching constants (KSV) of the four polymers to the analytes mainly agree with the spectral overlap area, suggesting the presence of a predominant resonance energy transfer mechanism. Figure 4 shows that the greatest spectral overlap is between the absorbance spectrum of 2-NA and the emission spectrum of HCP-1, which explains the high KSV value in fluorescence quenching.29,30 HCP-1, which has the bluest maximum emission peak, has the greatest spectral overlap with all of the analytes, revealing the best sensing efficiencies. PPE-2 has the smallest overlap, resulting in the poor sensing efficiencies. The spectra of HCP-2 and PPE-1 are close to each other. To 2-NA, PPE-1 has a larger overlap (402 nm -600 nm) than HCP-2. However, to other analytes, the overlap is less ideal, as the analytes have shorter absorption wavelengths than 2-NA. As to DNT, TNT and DNB, the overlap of their absorption 9 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 24

spectra are negligible with the emission spectra of polymers. In these cases, excited state electron transfer (between the electron-deficient nitroaromatic and the electron-rich polymers) may play a more important role during quenching process.20 The lower the LUMO energy of the analytes, the more efficient the quenching should be. The energy level of the analytes were calculated at B3LYP/6-311++G** level of theory (Table 2). The LUMO value of 3-NA, NP and 2-NA are inconsistent with their performances, suggesting the electron transfer mechanism is not the dominant interaction during the quenching process, but as explained above (Figure 4) energy transfer plays a role.

Table 2. Calculated Energy Level of the Analytes at B3LYP/6-311++G** Level of Theory. The Analytes Are Ordered According to Their Calculated LUMO-Level

Analyte A

2-NA

3-NA

NB

NP

DNT

DNB

TNT

PA

LUMO[eV]

-0.37

-2.67

-2.75

-2.92

-3.19

-3.40

-3.45

-3.91

-4.32

HOMO[eV]

-5.78

-6.48

-6.52

-7.95

-7.22

-8.44

-8.35

-8.84

-8.62

Instead of a single response of a specific polymer to one analyte, these four polymers

make

up

sensor

arrays,

recognizing

and

discriminating

the

nitro-analytes.3,31,32 We discriminated the nine analytes using our four polymers to build response patterns. The fluorescent polymers (0.03 µM) were quenched by the analytes (0.3 mM) in six replicates (the fluorescence response patterns are shown in


Page 11 of 24

Figure S2). The fluorescence intensity changes were recorded and analyzed by linear discriminant analysis (LDA), and four canonical factors were generated (62%, 28%, 6% and 4%). The two larger canonical factors give a 2-D discrimination plot with nine distinct clusters, well resolved without overlap (Figure 5). Especially, 2-NA and PA are well separated from the others.

0.8

0.6

0.4

0.2

0.0 300

1.0

0.8

0.6

0.4

0.2

Normalized Emission

A NB 3-NA DNT DNB TNT NP PA 2-NA HCP-1 HCP-2 PPE-1 PPE-2

1.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0 400

500

600

Wavelength (nm)

Figure 4. Spectra overlap between normalized absorbance spectra of nitro analytes (colorful lines) and emission spectra of polymers (black and grey lines).



-60

-30

0

30

2-NA

20

20 A

Factor 2 (28%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 24

10

10 NB DNT

PA

0

0 3-NA

-10

-20

-10

DNB

TNT

-20

NP

-60

-30

0

30

Factor 1 (62%)

Figure 5. 2-D canonical score plot of discriminant scores with 95% confidence ellipses for all obtained data points against different analytes. Factor 1 represents the overall ability of the analyte to quench the fluorescence of conjugated polymers.

For further study, all nine samples were tested randomly for four times. Detection and identification of the unknown samples were carried out by the LDA training matrix from our polymer sensor array. The response of polymers to each unknown sample was compared with the value of the classification data. With a 95% confidence interval, 100% accuracy can be obtained by this polymer sensor array.


Page 13 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Formation of F127 micelles, addition of conjugated polymers into the micelles to form fluorescent micelles and their schematic quenching process in water.

Detection of nitroaromatic analytes in water is attractive. Amphiphilic polymers encapsulate hydrophobic fluorescent molecules into the hydrophobic cores of the amphiphilic

polymer

micelles.33

Pluronic

F-127

(triblock

copolymers

poly(oxyethylene)-block-poly(oxypropylene)-block-poly(oxyethylene)),

of

non-toxic,

low cost self-assembles in water into nano-micelles. With F-127 as carrier and HCPs and PPEs as sensing cores, we detect nitroaromatics in water (Figure 6). The polymers dissolved in THF were dropped into the micellar solution of F-127 in water.34 The mixtures were treated in an ultrasonic bath for 1 h, THF was removed by vacuum distillation, resulting in the final micellar preparations (HCP-1-M, HCP-2-M, PPE-1-M and PPE-2-M) with F-127 at a concentration of 10 mg/L and the HCPs and PPEs at 0.5 µM.


HCP-1 M Abs HCP-1 M Em 1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 400

500

600

1.0

HCP-2 M Abs HCP-2 M Em 1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0

300

400

1.0

PPE-1 M Abs PPE-1 M Em 1.0

(d)

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2


0.0

0.0

300

400

500

600

Normalized Emission

(c)

500

600

Wavelength (nm)

Wavelength (nm)

700

1.0

PPE-2 M Abs PPE-2 M Em 1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 400

Wavelength (nm)

500

600

Normalized Emission

300

(b) Normalized Absorbance

1.0

Normalized Emission


(a)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 24

Normalized Emission


700

Wavelength (nm)

Figure 7. Normalized absorption and fluorescence spectra of (a) HCP-1-M, (b) HCP-2-M, (c) PPE-1-M and (d) PPE-2-M.

The photophysical properties of HCP micelles (HCPs-M) and PPE micelles (PPEs-M) are shown in Figure 7 and Table S4. The maximum emission peaks of the fluorescent micelles are red shifted compared to that of the original polymers, suggesting the formation of aggregates and / or planarization of the backbones. The fluorescence quantum yields (ΦF) for HCP-1-M, HCP-2-M, PPE-1-M and PPE-2-M are 12%, 17%, 19% and 20% in water, lower than their ΦF in THF (Table 1).


Page 15 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


We visualized the fluorescent micelles by scanning electron microscopy (SEM); images of F-127, HCP-1 and HCP-1-M in water are shown in Figure 8 (for other images see the supporting information, Figure S4). For F-127 micelles in water, small round particles were observed (Figure 8a). Without amphiphilic surfactant F-127, polymer HCP-1 (Figure 8a, prepared using the same procedure for HCP-1-M) forms a fibrous morphology. After the formation of the fluorescnt micelles, HCP-1-M shows larger average particle size compared to the F-127 alone, HCP-1 is trapped in the hydrophobic core of the F-127 micelles. F-127 is essential to aggregate HCP-1 in the micelles but also their proper dispersion in water.

Figure 8. SEM images of (a) F127, (b) HCP-1 and (c) HCP-1-M in water, scale bars shown are 500nm.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 24

HCPs-M and PPEs-M show increased quenching effects for the nitroaromatic analytes, as compared to those in organic solvents such as THF. The KSV values of the HCPs-M and PPEs-M are shown in Figure 9 and Table S4. From Figure 9 left, the fluorescence quenching efficiencies of the fluorescent micelles for nitro-analytes were in the same order of 2-NA > PA > NP > TNT > DNB > DNT > 3-NA > NB > A as their original polymers. HCPs-M show better quenching efficiencies to the analytes than PPEs-M, consistent with the performances of their polymers. The KSV of the fluorescent micelles HCPs-M are doubled (Figure 9, right) in comparison to the values of the HCPs. The limits of detection (LOD) of HCPs and their micelles for the analytes were estimated and listed in Table 3. The LOD of HCP-1-M to 2-NA in water is as low as 18 ppm, which is one third of the LOD of HCP-1 for 2-NA in THF. The HCP micelles are more sensitive towards nitroaromatics than the HCPs in THF.

The HCPs and PPEs are trapped in the micelles and in an aggregated phase; spatial confinement aggregates them together.35 The inner space of micelles provide an environment where the fluorescent polymers are close to each other and also the neighboring analytes. Thus, long-range exciton migration may happen between the analytes and the polymers. In addition, hydrophobic interior will lead to a pre-concentration of the hydrophobic analytes in the micelles or the interface to the aqueous solvent.36


Page 17 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Limits of Detection (LOD) for HCPs-M and HCPs Towards Analytes analyte

HCP-1-M [mol/L]

A

4.7 x 10-5

1.0 x 10-4

1.2 x 10-4

5.5 x 10-4

NB

2.4 x 10-6

8.3 x 10-6

6.8 x 10-5

1.2 x 10-4

3-NA

1.4 x 10-6

8.4 x 10-6

4.1 x 10-5

1.3 x 10-4

DNT

9.5 x 10-7

4.2 x 10-6

8.6 x 10-6

1.7 x 10-5

DNB

5.5 x 10-7

1.7 x 10-6

3.3 x 10-6

1.3 x 10-5

TNT

5.3 x 10-7

1.6 x 10-6

2.8 x 10-6

9.4 x 10-6

NP

5.2 x 10-7

1.5 x 10-6

2.2 x 10-6

5.1 x 10-6

PA

2.8 x 10-7

9.0 x 10-7

1.2 x 10-6

2.4 x 10-6

2-NA

1.8 x 10-7

9.0 x 10-7

5.0 x 10-7

1.3 x 10-6

HCP-2-M [mol/L] HCP-1 [mol/L] HCP-2 [mol/L]

20000 15000 10000 5000 HCP-1-M HCP-2-M PPE-1-M PPE-2-M

HCP-1-M HCP-2-M HCP-1 HCP-2

A NB 3-NA DNT DNB TNT NP PA 2-NA

Figure 9. Fluorescence quenching efficiencies of HCPs-M and PPEs-M for different analytes (left) and fluorescence quenching efficiencies of HCP-1, HCP-2, HCP-1-M and HCP-2-M for different analytes (right). The z-axis denotes the Stern-Volmer constant KSV. 17 ACS Paragon Plus Environment


Linear discriminant analysis was carried out with the quenching results obtained by the fluorescent micelles (0.5 µM) quenched by the analytes (0.3 mM, six replicates). The fluorescence intensity changes were recorded and analyzed by linear discriminant analysis (LDA); four canonical factors were generated (76%, 10%, 8% and 6%). The two larger canonical factors construct a 2-D discrimination plot with nine distinct clusters. As illustrated in Figure 10, the nine clusters are well separated without overlap. Even though the nine clusters are closer than in Figure 5, the unknown samples were identified with 100% accuracy based on this fluorescent micelle sensor array. This suggests that we not only can sense but also discriminate the nitroaromatics in water, which in principle should also allow to analyze where the nitroarenes originate from explosives or as industrial pollution. -100

0

100

PA

2-NA

A

10

Factor 2 (10%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 24

NB

0

10

0

TNT

DNB

NP

-10

-10 3-NA DNT

-100

0

100

Factor 1 (76%)

Figure 10. 2-D canonical score plot of discriminant scores with 95% confidence ellipses for all obtained data points of the fluorescent micelles against different 18 ACS Paragon Plus Environment

Page 19 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


analytes. Factor 1 represents the overall ability of the analyte to quench the fluorescence of the emitting micelles.

3. CONCLUSIONS

In summary, we have synthesized two truxene-based polymers (HCP-1 and HCP-2) and compared their sensory responses to those of the analogous poly(para-phenyleneethynylene)s (PPE-1 and PPE-2) with alkyl and alkyloxy chains. HCPs with a truxene moiety reveal better fluorescence response to the nitroaromatic analytes than linear PPEs: a) the presence of the electron rich truxene unit and b) the hyperbranched character lead to higher sensing efficiencies in explosive detecting even though the observed KSV are only in a medium range. The alkyl and alkyloxy side chains make a difference in their sensing performance. The quenching efficiency of the analytes correlates with a) the overlap area of the absorption spectra of analytes and the emission spectra of polymers, suggesting contributions of Förster energy transfer; b) in case of TNT, PA etc. electron transfer between the electron-deficient nitroarene and the electron-rich polymers based on the calculated energy level. The four polymers build up a sensor array which successfully discriminates the nine nitroaromatic analytes and identifies them with 100% accuracy. The fluorescent micelles form by addition of a concentrated THF solution of the conjugated polymers into an aqueous solution of amphiphilic F-127 micelles. These fluorescent HCP- and PPE-micelles efficiently detect and discriminate nitroarenes in water by a fluorescent



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 24

micelle sensor array based upon the four micelles-polymer element. In future we will manipulate the structure of the HCPs further to increase sensitivity and selectivity of the array to detect degradation products of explosives and land mines.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publication website at DOI: General information, the protocols to prepare HCP-1, HCP-2, PPE-1 and PPE-2, full spectra overlap between normalized absorbance spectra of nitro analytes and emission spectra of polymers, corresponding analytical data, table of KSV constants in THF and other solvents, images of aqueous suspensions of polymers HCP-2, PPE-1 and PPE-2 alone (a, c, e) with micelle suspensions of polymers in F-127 (b, d, f), table of KSV constants of the fluorescent micelles, fluorescence response pattern, canonical score plot and limit of detection of the PPEs and the micelles to analytes.

AUTHOR INFORMATION Corresponding Author *Fax +49(6221)548401; E-mail [email protected] (U.H.F.B.).

Notes The authors declare no competing financial interest. 20 ACS Paragon Plus Environment

Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ACKNOWLEDGMENTS W. H. and J. H thanks the CSC (Chinese Scholarship Council) for a fellowship.

REFERENCES (1) Xu, B. W.; Wu, X. F.; Li, H. B.; Tong, H.; Wang, L. X. Selective Detection of TNT and Picric Acid by Conjugated Polymer Film Sensors with Donor-Acceptor Architecture. Macromolecules 2011, 44, 5089-5092. (2) Purohit, V.; Basu, A. K. Mutagenicity of Nitroaromatic Compounds. Chem. Res. Toxicol. 2000, 13, 673-692. (3) Wu, J. T.; Tan, C. Y.; Chen, Z. F.; Chen, Y. Z.; Tan, Y.; Jiang, Y. Y. Fluorescence Array-Based Sensing of Nitroaromatics Using Conjugated Polyelectrolytes. Analyst 2016, 141, 3242-3245. (4) Nagarkar, S. S.; Desai, A. V.; Ghosh, S. K. A Fluorescent Metal–Organic Framework for Highly Selective Detection of Nitro Explosives in The Aqueous Phase. Chem. Commun. 2014, 50, 8915-8918. (5) Caygill, J. S.; Davis, F.; Higson, S. P. Current Trends in Explosive Tetection Techniques. Talanta 2012, 88, 14-29. (6) Caygill, J. S.; Collyer, S. D.; Holmes, J. L.; Davis, F.; Higson, S. P. Disposable Screen-Printed Sensors for The Electrochemical Detection of TNT and DNT. Analyst 2013, 138, 346-352. (7) Moore, D. S. Instrumentation for Trace Detection of High Explosives. Rev. Sci. Instrum. 2004, 75, 2499-2512. (8) Deshmukh, A.; Bandyopadhyay, S.; James, A.; Patra, A. Trace Level Detection of Nitroanilines Using A Solution Processable Fluorescent Porous Organic Polymer. J. Mater. Chem. C 2016, 4, 4427-4433. (9) Sohn, H.; Sailor, M. J.; Magde, D.; Trogler, W. C. Detection of Nitroaromatic Explosives Based on Photoluminescent Polymers Containing Metalloles. J. Am. Chem. Soc. 2003, 125, 3821-3830. (10) Yang, J.-S.; Swager, T. M. Porous Shape Persistent Fluorescent Polymer Films: An Approach to TNT Sensory Materials. J. Am. Chem. Soc. 1998, 120, 5321-5322. (11) Yang, J.-S.; Swager, T. M. Fluorescent Porous Polymer Films as TNT Chemosensors: Electronic and Structural Effects. J. Am. Chem. Soc. 1998, 120, 11864-11873. (12) Snow, E.; Perkins, F.; Houser, E.; Badescu, S.; Reinecke, T. Chemical Detection with A Single-walled Carbon Nanotube Capacitor. Science 2005, 307, 1942-1945.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 24

(13) Shanmugaraju, S.; Joshi, S. A.; Mukherjee, P. S. Self-Assembly of Metallamacrocycles Using A Dinuclear Organometallic Acceptor: Synthesis, Characterization, and Sensing Study. Inorg. Chem. 2011, 50, 11736-11745. (14) Kartha, K. K.; Babu, S. S.; Srinivasan, S.; Ajayaghosh, A. Attogram Sensing of Trinitrotoluene with A Self-Assembled Molecular Gelator. J. Am. Chem. Soc. 2012, 134, 4834-4841. (15) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Conjugated Polymer-Based Chemical Sensors. Chem. Rev. 2000, 100, 2537-2574. (16) Bunz, U. H. F.; Seehafer, K.; Bender, M.; Porz, M. Poly (aryleneethynylene) s (PAE) as Paradigmatic Sensor Cores. Chem. Soc. Rev. 2015, 44, 4322-4336. (17) Hu, R.; Lam, J. W.; Liu, J.; Sung, H. H.; Williams, I. D.; Yue, Z.; Wong, K. S.; Yuen, M. M.; Tang, B. Z. Hyperbranched Conjugated Poly (tetraphenylethene): Synthesis, Aggregation-Induced Emission, Fluorescent Photopatterning, Optical Limiting and Explosive Detection. Polym. Chem. 2012, 3, 1481-1489. (18) Narayanan, A.; Varnavski, O. P.; Swager, T. M.; Goodson, T. Multiphoton Fluorescence Quenching of Conjugated Polymers for TNT Detection. J. Phys. Chem. C 2008, 112, 881-884. (19) Liu, Y.; Mills, R. C.; Boncella, J. M.; Schanze, K. S. Fluorescent Polyacetylene Thin Film Sensor for Nitroaromatics. Langmuir 2001, 17, 7452-7455. (20) Xu, B.; Wu, X.; Li, H.; Tong, H.; Wang, L. Selective Detection of TNT and Picric Acid by Conjugated Polymer Film Sensors with Donor–Acceptor Architecture. Macromolecules 2011, 44, 5089-5092. (21) Cavaye, H.; Shaw, P. E.; Wang, X.; Burn, P. L.; Lo, S.-C.; Meredith, P. Effect of Dimensionality in Dendrimeric and Polymeric Fluorescent Materials for Detecting Explosives. Macromolecules 2010, 43, 10253-10261. (22) Cao, X.-Y.; Zhang, W.-B.; Wang, J.-L.; Zhou, X.-H.; Lu, H.; Pei, J. Extended π-Conjugated Dendrimers Based on Truxene. J. Am. Chem. Soc. 2003, 125, 12430-12431. (23) Lee, D.-c.; Artero, E. G.; Sui, X.; Blair, S. N. Review: Mortality Trends in The General Population: The Importance of Cardiorespiratory Fitness. J. Psychopharmacol. 2010, 24, 27-35. (24) Li, X.; Yu, Y.; Ji, Q.; Qiu, L. Targeted Delivery of Anticancer Drugs by Aptamer AS1411 Mediated Pluronic F127/Cyclodextrin-Linked Polymer Composite Micelles. Nanomedicine: NBM 2015, 11, 175-184. (25) Sonogashira, K.; Tohda, Y.; Hagihara, N. A Convenient Synthesis of Acetylenes: Catalytic Substitutions of Acetylenic Hydrogen with Bromoalkenes, Iodoarenes and Bromopyridines. Tetrahedron Lett. 1975, 16, 4467-4470. (26) Bender, M.; Seehafer, K.; Findt, M.; Bunz, U. H. F. Pyridine-based Poly (aryleneethynylene) s: A Study on Anionic Side Chain Density and Their Influence on Optical Properties and Metallochromicity. RSC Adv. 2015, 5, 96189-96193. (27) Phillips, R. L.; Miranda, O. R.; Mortenson, D. E.; Subramani, C.; Rotello, V. M.; Bunz, U. H. F. Gold Nanoparticle-PPE Constructs As Biomolecular Material 22 ACS Paragon Plus Environment

Page 23 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Mimics: Understanding The Electrostatic and Hydrophobic Interactions. Soft Matter 2009, 5, 607-612. (28) Nagarkar, S. S.; Joarder, B.; Chaudhari, A. K.; Mukherjee, S.; Ghosh, S. K. Highly Selective Detection of Nitro Explosives by A Luminescent Metal–Organic Framework. Angew. Chem. Int. Ed. 2013, 125, 2953-2957. (29) Thomas, S. W.; Joly, G. D.; Swager, T. M. Chemical Sensors Based on Amplifying Fluorescent Conjugated Polymers. Chem. Rev. 2007, 107, 1339-1386. (30) Hu, Z.; Lustig, W. P.; Zhang, J.; Zheng, C.; Wang, H.; Teat, S. J.; Gong, Q.; Rudd, N. D.; Li, J. Effective Detection of Mycotoxins by A Highly Luminescent Metal–Organic Framework. J. Am. Chem. Soc. 2015, 137, 16209-16215. (31) Miranda, O. R.; You, C.-C.; Phillips, R.; Kim, I.-B.; Ghosh, P. S.; Bunz, U. H. F.; Rotello, V. M. Array-Based Sensing of Proteins Using Conjugated Polymers. J. Am. Chem. Soc. 2007, 129, 9856-9857. (32) Han, J.; Wang, B.; Bender, M.; Seehafer, K.; Bunz, U. H. F. Water-Soluble Poly (p-aryleneethynylene) s: A Sensor Array Discriminates Aromatic Carboxylic Acids. ACS Appl. Mater. Interfaces 2016, 8, 20415-20421. (33) Tian, J.; Chen, H.; Zhuo, L.; Xie, Y.; Li, N.; Tang, B. A Highly Selective, Cell-Permeable Fluorescent Nanoprobe for Ratiometric Detection and Imaging of Peroxynitrite in Living Cells. Chem. Eur. J. 2011, 17, 6626-6634. (34) Liang, Z.; Chen, H.; Wang, X.; Sun, R. F127/Conjugated Polymers Fluorescent Micelles for Trace Detection of Nitroaromatic Explosives. Dyes Pigm. 2016, 125, 367-374. (35)Rainò, G.; Stöferle, T.; Park, C.; Kim, H. C.; Chin, I. J.; Miller, R. D.; Mahrt, R. F. Dye Molecules Encapsulated in A Micelle Structure: Nano-Aggregates with Enhanced Optical Properties. Adv. Mater. 2010, 22, 3681-3684. (36) Chowdhury, A.; Mukherjee, P. S. Electron-Rich Triphenylamine-Based Sensors for Picric Acid Detection. J. Org. Chem. 2015, 80, 4064-4075.

TOC-GRAPHIC



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 24


Truxene-Based Hyperbranched Conjugated ... - ACS Publications

Recommend Documents