Preparation of Poly(styrene)-b-poly(acrylic acid ... - ACS Publications

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Preparation of Poly (styrene)-b-poly (acrylic acid)Coupled Carbon Dots and Their Applications Moon-Jin Cho, and Soo-Young Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04942 • Publication Date (Web): 27 Jun 2017 Downloaded from http://pubs.acs.org on June 28, 2017

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ACS Applied Materials & Interfaces

Preparation of Poly (styrene)-b-poly (acrylic acid)-Coupled Carbon Dots and Their Applications Moon-Jin Cho and Soo-Young Park* Department of Polymer Science & Engineering, Polymeric Nanomaterials Laboratory, School of Applied Chemical Engineering, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu 41566, Korea *Corresponding author: [email protected] Abstract Carbon dots (CDs) are covalently coupled to the polyacrylic acid (PAA) block of polystyrene-b-polyacrylic acid (PS-b-PAA). The produced amphiphilic PS-b-PAA-CD shows excellent solubility and micellar formation in selective solvents. Transmission electron microscopy study shows that the micelle of PS-b-PAA-CD in water (a PAA-selective solvent) has a strawberry-like shape with the CDs as patches on the PAA corona surface although that in toluene (a PS-selective solvent) has a reversed shape with the CDs in the core. The fluorescence intensity of PS-b-PAA-CD in water is approximately twice that in toluene because the close contacts between the CDs in the core of the micelle lead to decreased fluorescence intensity due to π-π interactions. Thus, PS-b-PAA-CD can be used in sensors via fluorescence quenching through morphology control of micelle by solvent selectivity. The PS-b-PAA-CD aqueous solution is successfully used as an fluorescent ink for ink-jet printing and fluorescent fillers for poly(methyl methacrylate) (PMMA) composites. The quantum yields of the CDs in pure CD, PS-bPAA-CD, and a PMMA/PS-b-PAA-CD composite were almost the same (~41%). Due to its good solubility in organic solvents and self-assembly property in solution, the application scope of PS-b-PAA-CD can be further expanded in the future.

Keywords: Carbon dot, block copolymer, composite, micelle, ink, selective solvent.

1 ACS Paragon Plus Environment


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1. Introduction

Carbon is generally considered to have weak fluorescence and low solubility in solvents like water. A new nanocarbon material, carbon dots (CD), has recently been focused on due to its good solubility in water, strong luminescence, abundance, low cost, and benign nature.1 This CD can be applied to a polymer composite system as a filler. Several CD/polymer composites have been developed for inducing fluorescence properties in polymer matrix. For example, poly(vinyl alcohol)(PVA)/CD composites showed enhanced fluorescence emission compared to their corresponding aqueous solutions due to the compression effect on the surface passivation layer in the CDs.2 The quaternary CDs have been used for intense and tunable blue-red emission of poly(ethylene glycol)(PEG)/CD.3 Several systems like poly(acrylamide) (PAM)/CD, poly(acrylic acid) (PAA)/CD, and poly(vinylpyrrolidone) (PVP)/CD have been tested as multicolour composites with luminescent CDs having high quantum yield and controllable emission wavelengths.4 However, CDs in composites have mostly been used with water-soluble polymers like PVA, PEG, PAM, PAA, and PVP due to its hydrophilic nature. When the CD composite is prepared by solution mixing with a matrix polymer that is only soluble in organic solvents (other than water), the CD should be modified to make it soluble in organic solvents. One of the methods for functionalizing CDs is to graft polymer chains on their surfaces.5 The grafted polymer chains on the CD surface can not only provide functionality but also good solubility in organic solvents, depending on the nature of the grafting polymer. CDs could be synthesized using both top-down and bottom-up methods. Examples of top-down methods include arc discharge,6 laser ablation,7 chemical oxidation in strong acid,8 and electrochemical synthesis,9 and those of bottom-up methods include -

high-temperature pyrolysis using microwaves,10,11 solvothermal technique,12 14 and simple thermal combustion -

of the organic precursor molecules.15 17 The bottom-up methods offer fast and simple access to the production of a wide variety of CDs with surface functionalities and tunable properties. In this study, the CDs were prepared by of a solvothermal method (bottom-up technique) with citric acid and ethylene diamine as suppliers of the major carbon framework and the doped nitrogen through amine groups, respectively.12,16,18

A

polycation-b-polysulfobetaine

block

copolymer,

poly-[2-(dimethylamino)

ethyl

methacylate]-b-poly[N-(3-(methacryloylamino) propyl)-N,N-dimethyl-N-(3-sulfopropyl) ammonium hydroxide] (PDMAEMA-b-PMPDSAH), was grafted from CD via surface-initiated atom transfer radical polymerization (ATRP) by Cheng and co-workers.1 The CD-grafted PDMAEMA-b-PMPDSAH integrated the functions of imaging and gene delivery, leading to potential applications in theragnostics. In this system, the zwitterionic PMPDSAH block, cationic PDMAEMA, and carbon-dot core respectively served as the outer shell protecting


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the vector against nonspecific interactions with serum components, a deoxyribonucleic acid (DNA) condensing agent, and a multicolor cell-imaging probe. In the present work, however, an amphiphilic block copolymer, poly(styrene)-b-poly(acrylic acid) (PS-b-PAA), was grafted to (not from) the surface of the produced CD for the first time.19 The produced CD-coupled PS-b-PAA (PS-b-PAA-CD) is amphiphilic due to the presence of the hydrophilic CD-PAA block and the hydrophobic PS block. This amphiphilic PS-b-PAA-CD can be formed into micelles in a selective solvent and the micellar structure was found to affect its fluorescence properties. The PSb-PAA-CD aqueous solution was further used as the ink for an ink-jet printer. It is also used as a filler in a PMMA matrix as a demonstration of its application to a composite system that needs the CD to be soluble in organic solvents. These studies suggest new pathways to expand the application of the CD in the composite and in the solution.

2. Experimental 2.1 Materials

Citric acid (Sigma-Aldrich, USA), ethylene diamine (Daejung, South Korea), tetrahydrofuran (THF) (Duksan, South Korea), toluene (Duksan, South Korea), poly(styrene-b-acrylic acid) (PS-b-PAA, Mn = 2.8k-b-10.0k, PDI = 1.19, Polymer source, Canada), Rhodamine 6G (Sigma-Aldrich, USA), N-hydroxysuccinimide (NHS) (SigmaAldrich, USA), N-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride (EDC·HCl) (Sigma-Aldrich, USA), pH buffer solutions (Samchun, South Korea), poly(methyl methacrylate) (PMMA) (MW = 90000, KumHo Petrochemical, Korea) were used as received without further purification. Dialysis tubing cellulose (molecular weight cut-off (MWCO) = 14,000 Da) and benzoylated (MWCO = 2,000 Da) membranes were purchased from Sigma-Aldrich, USA. Deionized (DI) water was used after purification on a reverse osmosis system (Pure RO, Romax, South Korea). The EDC·HCl:NHS aqueous solution was prepared by mixing EDC·HCl (0.1 M) and NHS (0.2 M) with magnetic stirring for 15 min at room temperature, after which the aqueous solution became transparent.

2.2 Preparation of CD

Citric acid (0.4 g, 0.002 M) and ethylene diamine (270 µL, 0.004 M) were dissolved in water (80 mL) with magnetic stirring for 5 min. The solution was transferred to a bench-top reactor (4566, MK science, South Korea) and reacted at 160 °C for 5 h. After the reaction, the reactor was cooled to room temperature by turning it



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off. The produced CD solution was dialyzed in water using a dialysis tube (MWCO = 2000 Da) for two days. The CDs in the tube were collected and freeze-dried with a freeze dryer (fd-1000, EYELA, Japan). The powder form of the CDs was used for further experiments.

2.3 Preparation of PMMA/PS-b-PAA-CD composite

A predetermined amount of the PS-b-PAA-CD solution in THF (1.03 mg/mL) was mixed with the PMMA solution in THF (3 mL, 0.3 mg/mL) with magnetic stirring. The amount of PS-b-PAA-CD against PMMA is denoted as ϕ. The PMMA/PS-b-PAA-CD nanocomposite film was prepared by evaporating the solvent in the petri dish covered with aluminium foil on the bottom for easy detachment. The completely dried film was taken off from the bottom.

2.4 Ink-jet printing The aqueous PS-b-PAA-CD (8 mg/mL) and CD solutions were filled in the empty cartridge and fluorescent images were printed on the filter paper (Advantec, # 2, Tokyo Roshi Kaisha) using an ink-jet printer (1110, HP Deskjet, USA). 2.5 Quantum yield measurements The fluorescence quantum yield (Φ) was calculated from Rhodamine 6G as a standard material. UV-Vis spectra of the CD and PS-b-PAA-CD aqueous solutions with different concentrations were obtained after subtraction of the solvent spectrum. The absorbances of the aqueous CD and PS-b-PAA-CD solutions were measured at 350 nm and that of Rhodamine 6G was measured at 530 nm. The UV-Vis spectra of the PS-b-PAA-CD solutions show maximum intensity at 350 nm with background from PS-b-PAA, such that the absorbance of CD from the PS-b-PAA-CD solution was measured above the background after subtraction of the absorbance of PS-b-PAA, as shown in Figure S5c. The fluorescence intensities of the aqueous CD and PS-b-PAA-CD solutions (excited at 350 nm) were measured by integration of the peak at 450 nm and that of Rhodamine 6G (excited at 530 nm) was measured by integration of the peak at 550 nm (Figure S5b). The slope of the integrated fluorescence intensity vs. absorbance plot was measured and compared with that of the Rhodamine 6G having Φ = 95% (Φst) to calculate the quantum yield with the following equation: Φ = Φst (95) × (slope of sample)/(slope of standard) %

2.6 Measurements


.20

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Hydrodynamic diameter of CD was measured using a dynamic light scattering (DLS) device (Scatteroscope I, KOne Nano, South Korea). The critical micelle concentrations (CMCs) of PS-b-PAA in water and toluene were measured from the intersection of the two straight lines with small and large slopes in the plot of the dynamic light scattering (DLS, Scatteroscope I, K-One Nano, South Korea) intensity as a function of PS-b-PAA concentration. The micellar structures were imaged using transmission electron microscopy (TEM, Titan G2 ChemiSTEM Cs Probe, FEI Company, Netherland, 200 kV). The TEM samples were dip-coated onto a 200mesh carbon-coated copper grid (G75, Ted Pella Inc., CA), followed by solvent evaporation at 24 °C. The carbon-coated copper grid was prepared by coating the grid with a thin collodion film followed by coating with carbon using a carbon coater (208 Carbon Coater, Cressington, Singapore). The collodion thin film was prepared by spreading a drop of the collodion solution on water and catching the thin film on the copper grid. 1H-NMR spectra of the PS-b-PAA and PS-b-PAA-CD were obtained at ambient temperature with a 400-MHz Bruker spectrometer (Germany) using deuterated dimethyl sulfoxide (DMSO-d6) as the solvent. The UV-Vis, photoluminescence (PL), and Fourier-transform infrared (FTIR) spectra were recorded on a UV-2401 PC spectrophotometer (Shimadzu, Japan), an RF-5301 PC (Shimadzu, Japan), and a Jasco FT/IR–620 spectrometer (ATR method, Jasco, Japan), respectively. The structure of the PMMA/PS-b-PAA nanocomposite was investigated using scanning electron microscopy (SEM, S-4800, Hitachi, Japan) after coating the surfaces that had been fractured in liquid nitrogen with Pt. Small-angle X-ray scattering (SAXS) measurements were carried out using the 4C SAXS II beamline (BL) of the Pohang Light Source II (PLS II) with 3 GeV power at the Pohang University of Science and Technology (POSTECH, Korea). A light source from an In-vacuum Undulator 20 (IVU20: 1.4 m length, 20 mm period) of the Pohang Light Source II storage ring was focused with a vertical focusing toroidal mirror coated with Rh and monochromatized with a Si (111) double-crystal monochromator (DCM), yielding an X-ray beam wavelength of 0.734 Å. The X-ray beam size at the sample stage was 0.1 (V) × 0.3 (H) mm2. A two-dimensional (2D) Rayonix SX 165 CCD detector (Rayonix, USA) was employed. The sample-to-detector distance (SDD) was fixed at 4.00 m. The magnitude of the scattering vector, q = (4π/ λ) sin θ, was 0.06 nm-1 < q < 1.5 nm-1, where 2θ is the scattering angle and λ is the wavelength of the Xray beam source. The scattering angle was calibrated using the polystyrene-b-polyethylene-b-polybutadiene-bpolystyrene (SEBS) block copolymer standard. The solution SAXS data after subtraction of the solvent scattering were analysed with a Nanofit software supplied by Bruker. For TEM and SAXS measurements, PSb-PAA and PS-b-PAA-CD were first dissolved in THF (3 vol%, neutral solvent), followed by the dropwise addition of a large amount of solvent (water or toluene) to the THF solutions to obtain the desired concentration. The “hard sphere model with polydispersity” was employed to analyse the radius of the spherical micelle.



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3. Results and discussion

3.1 Synthesis of CD

Scheme 1 shows the overall scheme for the preparation of PS-b-PAA-CD, including the CD synthesis and the CD coupling to PS-b-PAA. The synthesized CD has a diameter of 2.1 ± 1.6 nm with narrow size distribution, as shown in the DLS spectrum (Figure SI 1a). The TEM image (Figure SI 1b) also shows a well-defined crystal lattice with 3.1 ± 0.7 nm diameter, close to the DLS data and consistent with reported data.21 Clear lattice fringes are visible in the image with a lattice spacing of 2.5 Å, corresponding to the (100) planes of graphite, indicating that the obtained CD is a kind of graphite.22 The FTIR spectrum (Figure 1a) exhibits a C=O stretching band at 1638 cm-1 and an N-H bending peak at 1528 cm-1, indicating that amine and carboxylic groups are present. Thus, the CD prepared from citric acid and ethylene diamine by the solvothermal method has a diameter of 3.1 ± 0.7 nm and bears several functional groups like carboxylic and amine moieties. These functional groups can be further utilized for grafting polymers. Figure SI 1c shows the UV-Vis spectrum and fluorescence spectra of the aqueous CD solution (0.0043 mg/mL) excited at different wavelengths. The UV-Vis spectrum shows the π-π* and n-π* transitions at 250 and 350 nm, respectively, which are consistent with the reported transitions for CD synthesized from citric acid and ethylene diamine.4 The intensity of the fluorescence spectrum is strongly dependent on the excitation wavelength. Gan et al.23, 24 systematically studied the underlying mechanism of the dependence of emission on the excitation wavelength in graphene quantum dots. They claimed that the strong blue emission is associated with the carbon defect states formed during reduction, and the enhanced longwavelength tuning photoluminescence of the functionalized reduced graphene oxide arises from the sp2 cluster size effect. The maximum intensity is observed at an excitation wavelength of 360 nm, which is close to the wavelength of the n-π* transition. The wavelength (450 nm), at which the maximum intensity of the fluorescence spectrum is observed, is almost independent on the excitation wavelength. The wavelength at maximum fluorescence intensity can be independent or dependent on the excitation wavelength, depending on the amount of the NH2 groups passivated on the CD surface, which can be controlled by the temperature of the citric acid/ethylene diamine reaction. Lower reaction temperatures produce larger amounts of passivated NH2 groups.4 It is also reported that the wavelength at maximum fluorescence intensity is independent when the amount of the passivated NH2 groups is high.4 The reaction temperature we used was 160 °C, which is lower than the common reaction temperature of 200 °C, implying that the wavelength at maximum fluorescence


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intensity is independent of the excitation wavelength. The CDs with constant fluorescence colour under UV light are preferred in our study, such that the reaction temperature of 160 °C was chosen in our study.

O

OH

O

O

O

OH O

NH2

O OH

OH

NH2 OH

NH

OH Citric acid

160 ℃, 5 h

+

Polymerization -nH2O

NH2

OH

HN O

O

NHO

NH2

HN OH

HN O

Ethylene diamine

O O

NH2 HO

O

OH

NH2

HN

OH

OH Carbonization O -nH2O NH

O OH

O OH

HO

NH2

O HO

NH2

O

CD (N-dopped)

PS-b-PAA n

m

O

EDC:NHS 24 ℃, 3 h in water

H

l

O

NH

OH NH2

COOH

: CD

NH2

PS-b-PAA micelle

PS-b-PAA-CD

NH2

Scheme 1. Synthesis of PS-b-PAA-CD

*

(i) *

(ii)

* - solvent

δ (ppm)

(a)



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(b) Figure 1. (a) 1H NMR spectra of (i) PS-b-PAA and (ii) PS-b-PAA-CD and (b) FTIR spectra of (i) CD, (ii) PS-bPAA, and (iii) PS-b-PAA-CD.

3.2 Grafting PS-b-PAA on CD

Dialysis was performed in water for two days to remove the unreacted CDs. The free CDs are removed from the dialysis tube and the amount of the dialysate (free CDs in water) can be calculated from the fluorescence intensity of CD in solution. Figure SI 2 shows the plot of the fluorescence intensity (at 450 nm) as a function of CD concentration, when excited at 350 nm. The good linearity (R2=0.9997) of this plot allows it to be used as a calibration curve for the CD concentration. From this calibration curve, the amount of un-reacted CDs in solution was measured. The amount of CDs reacted with PS-b-PAA was calculated to be 28.5 wt% against PS-bPAA by subtracting the amount of the un-reacted CD from that of the input CD concentration. The number of CD coupled to a single PAA chain is calculated to be 1.54 assuming density = 2.267 g/cm3 (graphite density) and radius = 1.5 nm (data from TEM). Thus, we found from the fluorescence data of the dialysis solution that a large quantity of CDs was coupled to the PAA block. Attachment of CD to PS-b-PAA can be confirmed by 1H NMR spectroscopy. Figure 1a shows the 1H NMR spectra of PS-b-PAA and PS-b-PAA-CD. The NMR spectrum of PS-b-PAA shows that the protons of the


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benzene ring of the PS block appear at 7.06 and 6.57 ppm and those of the carboxylic groups of the PAA block appear at 12.25 ppm. These peaks are retained in the NMR spectrum of the PS-b-PAA-CD. Additionally, a new doublet peak due to an amide proton appears at 5.80 ppm, confirming the coupling between the carboxylic group and CD. Figure 1b shows the FTIR spectra of CD, PS-b-PAA, and PS-b-PAA-CD. The FTIR spectrum of the CD (Figure 1b (i)) exhibits the C=O stretching band at 1691 cm-1 and N-H bending peak at 1532 cm-1, confirming the presence of amine and carboxylic groups. The FTIR spectrum of PS-b-PAA (Figure 1b (ii)) shows peaks due to the hydrogen-bonded OH at ~3000 cm-1, C-O stretch at 1172 cm-1, C=O stretch at 1637 cm-1, and aromatic C=C stretch at 1456 cm-1, confirming the presence of PAA and PS blocks. The FTIR spectrum of PS-b-PAA-CD (Figure 1b (iii)) shows an additional N-H bending peak at 1532 cm-1 and a C-N stretching band at 1239 cm-1, also indicating that new amide bonds were formed between the carboxylic and amine groups. Thus, based on the NMR and FTIR data, we confirmed that the CD and PAA blocks were successfully coupled using an EDC:NHS agent.

3.3 Solubility of PS-b-PAA-CD

Figure 2a shows that the photographs of CDs under UV light at 365 nm in various solvents. The CDs in polar solvents like water, ethanol, and methanol show strong fluorescence, and those in other solvents like DMF, toluene, chloroform, ethyl acetate, and THF show weak fluorescence, while those in DMSO exhibit mid-range fluorescence. These results indicate that the produced CDs are soluble only in polar solvents and those in other solvents show reduced florescence intensity due to strong π-π interactions of their aggregates in poor solvents.25 Coupling PS-b-PAA to CD can improve the solubility of CD in organic solvents and accordingly improve the solution fluorescence. The solubility of PS-b-PAA-CD was tested in PS-selective, PAA-selective, and neutral solvents. DMSO, THF, and ethyl acetate were neutral solvents, water, ethanol, and methanol were PAAselective solvents, and chloroform, DMF, and toluene were PS-selective solvents.20 Figure 2b shows the photographs of the PS-b-PAA-CD solutions in several solvents under UV light at 350 nm. The solutions show little fluorescence in the PS-selective solvents (chloroform, DMF, and toluene), although they show strong fluorescence in the PAA-selective (water, ethanol, and methanol) and neutral (DMSO, THF, and ethyl acetate) solvents. The decreased fluorescence intensity from the solutions in the PS-selective solvents (chloroform, DMF, toluene) was confirmed from the fluorescence spectra of the PS-b-PAA-CD solutions (Figure SI 3). These results indicate that fluorescence of the PS-b-PAA-CD is strongly dependent on the selectivity of the used solvent. In


order to understand this fluorescence dependency on solvent selectivity, the structure of PS-b-PAA in solvent was studied using SAXS and TEM.

10

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interactions between CDs.25 Micellar structure can be studied using a solution SAXS method. Figure 3a shows the SAXS patterns of the PS-b-PAA in water and toluene (0.2 wt%). The typical SAXS pattern for a spherical particle was observed for micelles in both water and toluene. The diameters calculated from curve fitting with a hard sphere model were 18.0 and 16.2 nm for the PS-b-PAA micelles in water and toluene, respectively. The bigger diameter of the micelle in water than that in toluene is because the long chains (i.e. PAA) were included in the corona and were swelled by hydration. Figure 3b shows the SAXS patterns of PS-b-PAA-CD in water and toluene (0.2 wt%). We found that 28.5 wt% of CD against PS-b-PAA was coupled to PS-b-PAA in the PS-bPAA-CD, as mentioned before. The electron density of the graphitic CD was much higher than that of the organic PS-b-PAA; therefore, CDs are more visible in SAXS because the electron density difference is a source of X-ray scattering. The SAXS pattern of the PS-b-PAA-CD in toluene exhibits the same shape (that for a spherical micelle) as that of the PS-b-PAA, although a good model fitting was difficult to achieve with a hard sphere model, indicating a large size distribution of the spherical micelle. The size distribution of the PS-b-PAACD micelle in toluene was also observed in TEM, which will be discussed in the next section. However, the SAXS pattern of the PS-b-PAA-CD in water shows a micellar shape completely different from that of the PS-bPAA. The slope of the SAXS pattern at low q (0.065–0.12 nm) is 2.2 in log scale in both x and y axes, indicating that the produced micelle is a two-dimensional vesicle. The PS-b-PAA-CD micelle in water should be a hairy one due to the much longer PAA of the corona than the PS of the core so that the micelle cannot be vesicle but sphere. The vesicular shape obtained from the SAXS solution pattern is due to the high-electron-density CDs located on the corona. Thus, the CDs coupled with PAA block are located in the core and the corona in toluene and water, respectively. This location of the CDs was further confirmed from TEM studies.



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(a)

(b)


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Figure 3. SAXS patterns of (a) PS-b-PAA (0.2 wt%) and (b) PS-b-PAA-CD (0.2 wt%) in (i) water and (ii) toluene (the dotted and solid lines are experimental and curve-fitted data, respectively); the curves of (i) in (a) and (b) are vertically separated by × 10 from the curves of (ii). Figures 4a–c show the TEM images of the PS-b-PAA and PS-b-PAA-CD micelles in water. Uniform-sized PSb-PAA micelles (Figure 4a) with diameters of 30 ± 3 nm were observed. The micelles observed in the TEM images present the entire size of the dry micelle, while those observed in the solution SAXS exhibit mostly the micellar core because of the higher electron density of the core than the hydrated corona; a small part of the corona can be observed under X-ray irradiation due to the high electron density of the congested corona close to the core.25-26 Thus, the diameter of the micelles measured from the TEM images (30 nm) was larger than the value obtained from SAXS curve fitting (18 nm). For the PS-b-PAA-CD micelles (Figures 4b and c), distinctive patches of the CDs are visible on the micelle surface, consistent with the apparent vesicle shape obtained from the SAXS data. The insets show the same lattice layers of the CD as that of the pure CD, confirming that the patches on the surface are the coupled CDs (Figure 4g).

Figures 4d–f show the TEM images of the PS-b-PAA and PS-b-PAA-CD micelles in toluene. Uniform-sized PSb-PAA micelles (Figure 4d) with diameters of 36 nm were observed although a large size distribution was observed from PS-b-PAA-CD micelles reflecting the difficulty in curve fitting of SAXS data. The diameter of the PS-b-PAA micelles measured from the TEM images (36 nm) was larger than the value obtained from SAXS curve fitting (16 nm) due to the same reason as in water. For the PS-b-PAA-CD micelles (Figures 4e and f), the CDs are located in the core, as demonstrated by the dense population of the CD at the core. In toluene, the PAA and PS blocks would be present in the core and the corona of the micelle, respectively; therefore, the CDs attached to the PAA block would be in the micelle core (Figure 4h). Thus, the CDs of the PS-b-PAA-CDs in toluene are confined in the core and are arranged closer to each other than those in water, which leads to fluorescence quenching due to π-π interactions. This hypothesis was further tested with fluorescence spectroscopy.



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(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Figure 4. TEM images of (a, d) PS-b-PAA and (b, e) PS-b-PAA-CD micelles in (a, b) water and (d, e) toluene; water and THF represent PAA-selective and PS-selective solvents, respectively. (c) and (f) are the enlarged


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images of the area enclosed in boxes in (b) and (e), respectively. Micellar models of PS-b-PAA-CD in (g) water and (h) toluene. The scale bars of the enlarged TEM images in the insets in Figures 4a, b, d, and e are 10 nm. The fluorescence intensity of the PS-b-PAA-CD solution can be affected by micellar morphology. Figure 5a shows the fluorescence spectra of the PS-b-PAA-CD solutions in water (PAA-selective solvent), toluene (PSselective solvent), and THF (neutral solvent). The fluorescence spectra of the PS-b-PAA-CD solutions in water and THF had similar peak intensity, although that in toluene shows half the peak intensity of those in water and THF. The peak positions of the PS-b-PAA-CD solutions in water, THF, and toluene were 450, 444, and 437 nm, respectively, i.e. an increase in solvent polarity is accompanied by a red shift. As molecules of the polar solvent re-orient around the excited CD molecules (solvent relaxation), they assist in stabilizing and lowering the energy level of the CD excited state; this reduces the energy separation between the ground and excited states and causes red shift of fluorescent emission.27 Thus, the CDs confined in the micelle core show quenched fluorescence intensity due to the π-π interactions among the closely packed CDs in the core 25, indicating that the nature of the solvent is important for controlling the fluorescence property of the PS-b-PAA-CD solution. The nature of the solvent can be finely controlled by altering the mixing ratio of the mixture of two solvents. THF and toluene are neutral and PS-selective miscible solvents, respectively; therefore, solvents of different natures can be obtained by controlling the mixing ratio of the THF/toluene mixture. The toluene content in the mixture is denoted as ζ. Figure SI 4 shows the fluorescence spectra of the PS-b-PAA-CD solutions in solvents of different ζ values. Their fluorescence intensities at 450 nm are plotted as a function of ζ (Figure 4b). The fluorescence intensity linearly increases with decreasing ζ, indicating that the CDs in the core become progressively less confined with increasing ζ because of the decreased solvent selectivity. This result also indicates that PS-b-PAACD can be used as a sensor for determining the composition of a mixture by measuring the fluorescence intensity.



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(a)

(b) Figure 5. (a) Fluorescence spectra of the PS-b-PAA-CD solutions in water (PAA-selective solvent), toluene (PSselective solvent), and THF (neutral solvent) when excited at 350 nm, and (b) fluorescence intensity at 350 nm in the toluene/THF mixture as a function of volume fraction of THF (ζ) when excited at 350 nm.


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3.5 Ink-jet printing of PS-b-PAA-CD

PS-b-PAA-CD dissolved in water can be used as an ink for ink-jet printing. Figure 6a shows the KNU symbol printed on a filter paper using aqueous solutions of CD (8 × 0.285 = 2.28 mg/mL) and PS-b-PAA-CD (8 mg/mL) as the ink. The concentration was controlled to obtain the same CD concentration in both cases. The fluorescence images with both CD and PS-b-PAA-CD ink are visible under UV light at 365 nm, although both are invisible under white light. This indicates that these CD aqueous solutions can be used as fluorescence inks for an ink-jet printer. However, the resolution of the image printed with PS-b-PAA-CD is much better than that printed with CD, indicating that better and uniform dispersion of CD on the filter paper is achieved with PS-b-PAA-CD. Thus, we found that PS-b-PAA-CD is printable on paper with good resolution without the need for any additional ingredients.

Figure 6. Photographs of ink-jet printings of the fluorescent images on filter papers with aqueous solutions of (a) CD (2.28 wt%) and (b) PS-b-PAA-CD (8 wt%) with the same CD concentration.

3.6 PMMA/PS-b-PAA-CD nanocomposite

The solubility of the PS-b-PAA-CD in organic solvents can be applied for the preparation of nanocomposites with a matrix only soluble in organic solvents. The PS-b-PAA-CD can be dissolved in THF, which is also a good solvent for PMMA. PMMA is a transparent polymer and is used as a model matrix for a nanocomposite. Figures 7a and b show the photographs of the PMMA/PS-b-PAA-CD composite films prepared with different PS-bPAA-CD amounts (ϕs) under white and UV lights at 365 nm, respectively. The composite was prepared with ϕ ≤ 0.5 wt% in order to investigate the effect of the small amount of CD on the fluorescence property. The prepared



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films were almost transparent under white light, but were fluorescent under UV light. The fluorescence intensity of the composite film increases with increasing value of ϕ. The quantitative data of the transparency and fluorescence properties were studied using UV-Vis and fluorescence spectroscopies. Figure 7c shows the UVVis spectra of the prepared composites. The absolute transmittance values of the composites at 550 nm were 98.7%, 98.4%, 98.7%, 98.3%, 98.0%, and 97.3% for ϕ = 0, 0.08, 0.16, 0.25, 0.33, and 0.5 wt%, respectively, indicating that the prepared composites were almost transparent. Figure 7d shows the fluorescence spectra (excited at 350 nm) of the prepared composites. The PMMA film shows no fluorescence intensity, and the fluorescence intensities of the composites increase with increasing value of ϕ. The quantum yield of the CD in the PMMA/PS-b-PAA-CD was calculated to be quite high (~41%) based on these UV-Vis and fluorescence data and have been discussed in the next section. The morphologies of the PMMA/PS-b-PAA-CD composites were studied with their fractured surface. Figure 7e shows the SEM images of the fractured surfaces of the PMMA/PS-b-PAA-CD composite (ϕ = 0.5 wt%) and pure PMMA. The fractured surface of the PMMA film shows a smooth surface without any defects, while that of PMMA/PS-b-PAA-CD composite shows uniformsized particles with an average diameter of 40 nm without any aggregations. Thus, the high quantum yield of the PMMA/PS-b-PAA-CD composite was due to the good dispersion of the CD in the PMMA matrix, assisted by the PS-b-PAA.


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(c)

(d)



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(e) Figure 7. Photographs of the PMMA/PS-b-PAA-CD films under (a) white light and (b) UV light at 365 nm; (c) transmittance and (d) fluorescence spectra (excited at 360 nm) of the PMMA/PS-b-PAA-CD composites for ϕ = (i) 0, (ii) 0.08, (iii) 0.16, (iv) 0.25, (v) 0.33, and (vi) 0.5 wt%; (e) SEM images of the fractured surfaces of (i) PMMA and (ii) PMMA/PS-b-PAA-CD composite film (ϕ = 0.5 wt%).

3.7 Quantum yield of PS-b-PAA-CD and PMMA/PS-b-PAA-CD

Fluorescence quantum yields (Φs) of the CD, PS-b-PAA-CD, and PMMA/PS-b-PAA-CD were measured using Rhodamine 6G as a standard material. Figure 8 shows the fluorescence intensity plot as a function of the absorbance measured from the UV-vis and fluorescence spectra of the Rhodamine 6G, CD, PS-b-PAA-CD aqueous solutions, and the PMMA/PS-b-PAA-CD composites for different CD concentrations (Figure SI 5). The slopes of the fluorescence intensity vs. absorbance plots for Rhodamine 6G, CD, and PS-b-PAA-CD aqueous solutions were 488094, 224611, and 204868 (arbitrary units), which can be converted to fluorescence quantum yields of 95% (reported),28 42%, and 40%, respectively. The quantum yield of PS-b-PAA-CD is similar to that of CD, indicating that the fluorescence property of CD was retained upon coupling to PS-b-PAA. The quantum yield of CD in the composite was calculated by the same method as that used for PS-b-PAA-CD (using the data in Figures 7c and d). The calculated quantum yield is 41%, which is almost the same as those of CD and PS-bPAA-CD, indicating that the PMMA matrix did not interfere with the fluorescence property of the CD. The quantum yield of 41% in the composite is higher than reported values, indicating that PS-b-PAA-CD can be a better choice for a fluorescent filler in a composite.3 Thus, PS-b-PAA-CD broadens the application of fluorescent


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CDs as fluorescent fillers in composites and as fluorescent inks for ink-jet printers by increase of their solubility in organic solvents and their self-assembly characteristic in selective solvents, respectively.

Figure 8. Fluorescence intensities of the aqueous solutions of Rhodamine 6G, CD, and PS-b-PAA-CD aqueous solutions, and of PMMA/PS-b-PAA-CD composite films as a function of the measured absorbance which is controlled by the concentrations of the aqueous solutions and the amounts of PS-b-PAA-CD in the composite. The excitation wavelengths are 350 and 530 nm for the CD-containing materials and Rhodamine 6G, respectively.

4. Conclusion

28.5 wt% of CDs was successfully coupled to PS-b-PAA (number averaged molecular weight; 2.8k-b-10k g/mole) using an EDC:NHS coupling agent which brought about covalent bonding between –COOH groups of the PAA block and –NH2 groups of the CD. The produced PS-b-PAA-CD was formed into micelles in selective solvents. In PAA-selective solvents (e.g. water), the CDs were located in the corona of the micelle; therefore, the fluorescence property of the CDs was not deteriorated. The aqueous PS-b-PAA-CD solution was successfully tested as a fluorescent ink for an ink-jet printer. In PS-selective solvents (e.g., toluene), the CDs were located in the micelle core; therefore, the fluorescence would be quenched by π-π interactions due to the close contacts between the CDs. In a neutral solvent (e.g. THF), the PS-b-PAA-CD shows no decrease in fluorescence intensity



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compared to that in water. The THF solution of PS-b-PAA-CD was applied to solution blending with PMMA. The resultant PMMA/PS-b-PAA-CD composite film showed excellent fluorescence property with the same quantum yield as those of CD and PS-b-PAA-CD. Thus, the PS-b-PAA-CD broadens the application of fluorescent CDs in fields like sensors, fluorescent fillers in composites, and fluorescent inks for ink-jet printers, by increasing solubility in organic solvents and self-assembly in selective solvents. Several other applications are also possible with this fascinating PS-b-PAA-CD.

Supporting Information. DLS, TEM, UV-vis, and fluorescence spectra of the prepared CD, calibration curve for the calculation of CD concentration from the fluorescence spectra, fluorescence spectra of PS-b-PAA-CD in PS-selective, PAA-selective, and neutral solvents, fluorescence spectra of PS-b-PAA-CD in toluene/THF mixtures, and UV-vis and fluorescence spectra of PMMA/PS-b-PAA-CD composites.

Acknowledgements: This work was supported by the National Research Foundation of Korea (NRF2014R1A2A1A11050451)

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EDC/NHS CD

PS-b-PAA

PS

PAA-selective solvent

24°C, 3h

PS-b-PAA-CD

PS

PS-selective solvent


Preparation of Poly(styrene)-b-poly(acrylic acid ... - ACS Publications

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