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Visible Light-Driven Photocatalysts of Perfluorinated Silica-Based Fluorescent Carbon Dot/TiO2 for Tunable Hydrophilic-Hydrophobic Surfaces Young Kwang Kim, Eun Bi Kang, Sung Han Kim, Shazid Md. Sharker, Beyung Youn Kong, Insik In, Kang Dae Lee, and Sung Young Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12618 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 23, 2016

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ACS Applied Materials & Interfaces

Visible Light-Driven Photocatalysts of Perfluorinated SilicaBased Fluorescent Carbon Dot/TiO2 for Tunable Hydrophilic-Hydrophobic Surfaces Young Kwang Kim1, Eun Bi Kang2, Sung Han Kim1, Shazid Md. Sharker3, Beyung Youn Kong2, Insik In1, 4, Kang-Dae Lee5*, Sung Young Park1, 2*

1

Department of IT Convergence, Korea National University of Transportation, Chungju 380-702,

Republic of Korea. 2

Department of Chemical & Biological Engineering, Korea National University of

Transportation, Chungju 380-702, Republic of Korea. 3

Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST),

Daejeon 305-701, Republic of Korea. 4

Department of Polymer Science & Engineering, Korea National University of Transportation,

Chungju 380-702, Republic of Korea. 5

Department of Otolaryngology-Head and Neck Surgery, Kosin University College of Medicine,

262 Gamcheon-ro, Suh-gu, Busan, 49267, Republic of Korea.

ACS Paragon Plus Environment

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ABSTRACT In this study, a new hydrophilic–hydrophobic transition surface was designed via visible-lightinduced photocatalytic perfluorinated silica-based fluorescent carbon nanoparticles (FNPs)/TiO2. Perfluorinated silica–polydopamine hybrid FNPs (f-FNPs) were easily fabricated by carbonization in an emulsion system consisting of tetraethyl orthosilicate and dopamine, followed by the deposition of TiO2 on f-FNPs, which demonstrated the reversal from hydrophobic to hydrophilic nature during successful photocatalysis. The synergistic effect of silica–carbon and the deposited TiO2 NPs led to the decomposition of methylene blue under UV and visible light irradiation, demonstrating that FNPs/TiO2 sustains photocatalytic activity. The profound contact angle with the catalytic kinetics curve and precise morphology and extension of cells detach antifouling exceptionally unrestricted the synergistic effect of silica-carbon on TiO2 NPs on a coated paper substrate. Given the interest in the manipulation of hydrophobicity and hydrophilicity, this study can serve as a guideline for the fabrication of photocatalytic surfaces where water spreads completely.

KEYWORDS: hydrophilic–hydrophobic surface, photocatalysis, UV–vis, TiO2, silica–carbon dot.


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1. INTRODUCTION Carbon and silica-based fluorescent materials with metal nanoparticles (NPs) have been widely utilized for fluorescence sensing, which exploits the photodynamic effect, as well as in optoelectronic devices, where conversion of energy for photocatalytic activity,1 attributed to their superior structural integrities, stability, durability, and biocompatibility, in addition, because of flexibility in fabrication, they can meet the requirements for different applications.2,

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The

advanced design of NPs exhibiting tunable hydrophobic and hydrophilic properties can broaden the scope of research areas, such as surface functionalization, surface patterning, and antifouling, which in turn can make them suitable for demanding applications.4 The post-deposition of metal oxide NPs on such carbon and silica-based NPs, as well as interaction between them, have resulted in enhanced durability.5 In particular, in the last few decades, titanium dioxide NPs (TiO2 NPs) have achieved tremendous attention because of their unique photoelectric and antiomniphobic properties.6, 7 On the other hand, silicon is an attractive material widely used in the microelectronics industry. Because of the quantum confinement effect of electron–hole pairs, silicon crystals or quantum dots (SiQDs) exhibit fluorescence emission, with an appreciable photoluminescence (PL) quantum yield.8, 9 Typically, photoluminescent SiQDs are prepared with hydrogen, halogen, or oxide-terminated surfaces. Because of the lack of a lattice-matched semiconductor barrier layer, surface properties of SiQDs play a crucial role in the characteristic photophysics of SiQDs.9 In this context, different methods have been reported for the synthesis of SiQDs, particularly using tetraethyl orthosilicate (TEOS) and silicon tetrachloride (SiCl4) in a reducing medium, affording NPs exhibiting blue luminescence. Typically, perfluorinated triethoxysilane (f-silane) has been used as the additive for silicon NPs for assisting in surface modification, which renders anti-


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adhesive behavior to polar and non-polar substances with omniphobicity.10, 11 In particular, by the introduction of optical interferences on Si NPs, fluorescence intensity is an important issue for emission. Furthermore, it exhibits very good weathering stability, attributed to the carbon– fluorine bond, and polycondensation renders hydrophobicity. As a result, this wetting property ensures the protection of metal substrates from corrosion. In surface chemistry research, dopamine (DA)-containing materials have been widely investigated as the adhesion layer for substrate fabrication.12, 13 Recently, DA has also been used as the carbon source for synthesizing yolk–shell carbon spheres. In that study, distinct D- and Gbands are observed in the Raman spectra, corresponding to a graphite-like nanostructure of carbonized DA. Carbonized polydopamine (pDA) with 40–50 stacking layers with thicknesses of 15 nm is speculated to demonstrate considerable potential in the field of optoelectronics.14 Furthermore, carbon-derived fluorescent NPs (FNPs) exhibit photoluminescence properties, attributed to either delocalized electrons or increased sp2 hybrization.15 Previous studies have reported the effectiveness of a hydrophobic–hydrophilic surface in cells for mediating adhesion. In addition, the use of these surfaces have demonstrated improvement in biomedical applications, e.g., the use of superhydrophilic microspots for screening cells, polyacrylamide brushes for controlling cell and protein retention, and 2D scaffold for immobilizing site-selective cells.16, 17, 18 Nevertheless, these studies have employed the use of patterned substrates and have not focused on the photocatalytic surface over which water spreads completely. In this study, we designed a hydrophilic–hydrophobic transition surface via the synthesis of visible-light-induced photocatalytic perfluorinated silica-based fluorescent carbon nanoparticles (f-FNPs/TiO2). The synergistic effect of silica-carbon and deposited TiO2 resulted in improved photocatalysis. The conversion of a high-surface-energy hydrophobic surface to a


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low-surface-energy hydrophilic surface

caused

by visible-light-induced

photocatalytic

perfluorinated silica-based fluorescent carbon NPs in response to photocatalysis was explored. Furthermore, the contact angle kinetic curves and photocatalytic performance were utilized to demonstrate that these silica-core-carbon deposited TiO2. Hence, this study indicates that the benefits attained from the intervention of f-FNPs/TiO2 can possibly address advanced requirements of photocatalysis across a wide range of applications.

2. EXPERIMENTAL SECTION 2.1 Materials and characterization Trizma base, Trizma HCl, dopamine hydrochloride (DA), TEOS, 1H,1H,2H,2Hperﬂuorodecyl triethoxysilane (f-silane), TiO2 (anatase nanopowder, 400 nm) was used. 2.2 Synthesis of perfluorinated dopamine-induced silica–carbon hybrid NPs (f-silica/pDA) Silica–carbon hybrid NPs were prepared by the reduction of DA under mild alkaline conditions. Briefly, 8 mmol of TEOS and 2 mmol f-silane were first mixed with 0.53 mmol of DA in 20 mL ethanol. Second, the resulting mixture was allowed to react for another 12 h at room temperature. Next, the crude product was purified by dialysis (molecular weight cut-off of 1,000) against water for 24 h and then freeze-dried, affording a pure product. 2.3 Carbonization of perfluorinated dopamine-induced silica–carbon hybrid NPs using an acid catalyst (f-FNPs) Carbonization was performed according to a previously published procedure.15 First, fsilica/pDA (1 g) was dissolved in 5 mL of water, followed by the addition of 10 mL of H2SO4 (36 N). The reaction mixture was allowed to react for another 10 min at room temperature with continuous magnetic stirring. After 10 min, the pH was neutralized with NaOH (1 M), and the crude product was purified by dialysis (molecular weight cut-off of 1000) against water and then freeze-dried, affording perfluorinated fluorescence silica-based carbon NPs (f-FNPs). 2.4 Synthesis of TiO2-integrated f-FNPs (f-FNPs/TiO2) For synthesizing TiO2-integrated f-FNPs from perfluorinated fluorescence silica-based carbon NPs, f-FNPs (1 g) were dissolved in 5 mL of a Tris buffer solution (10 mM, pH = 8.5) and stirred at room temperature for 12 h. Catechol reduction was confirmed by the visual


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evaluation of the dark gray color. Then, TiO2 (0.3 g) was dissolved in 30 mL of distilled water in a 250 mL flask. The above two solutions were mixed and vigorously stirred to form a homogeneous solution. Upon reaction completion, the solution was centrifuged at 4,000 rpm for 10 min, affording perfluorinated fluorescence silica-based carbon NPs/TiO2 (f-FNPs/TiO2). 2.5 Cell attachment study of surface-coated f-FNPs/TiO2 For evaluating cell attachment and antifouling, HeLa cells were grown on the surface of cellulose paper. This paper was prepared by the Meyer-rod coating of f-FNPs/TiO2 (5 mg/mL). Briefly, 1 mL of an ethanol solution of f-FNPs/TiO2 was dropped on the edge of the paper substrate, and a Meyer rod (8 mm) was pulled over the solution, resulting in a uniform wet film. This film was then dried with flowing air under heating at 80 °C for 3 s. At the end of coating, the cells were cultured (1 × 105 cells/mL) using an appropriate growth medium in a CO2 incubator. The cells were exposed to UV and visible light at different times (0, 0.5, 1, 3, 6, 12, 24, and 60 h). Finally, the cells were counted using an optical microscope, which were plotted against light irradiation.

3. RESULTS AND DISCUSSION Polydopamine-mediated silica–carbon-based UV–vis light-responsive photocatalytic NPs were fabricated, independent of material, affording a hydrophobic–hydrophilic transition surface with TiO2 NPs. Scheme 1 outlines the schematic of the overall approach. First the catehol groups of pDA possibly accelerate the hydrolysis and condensation of TEOS and f-silane, resulting in the formation of silica–pDA hybrids (f-silica/pDA).19 Perfluorinated fluorescence silica-based carbon NPs (f-FNPs) were formed by the dehydration and carbonization of f-silica/pDA using a


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strong acid catalyst. Finally, f-FNPs were assembled with TiO2 NPs (f-FNPs/TiO2) under mild alkaline pH by exploiting the adhesive property of DA.20 This new route to prepare UV–vis light-responsive photocatalytic silica–carbon NPs by the adhesive deposition of TiO2 can significantly enhance energy conversion and exhibit hydrophilic–hydrophobic reversal behavior while retaining electrochemical kinetics and durability (Scheme 1). 3.1. UV–vis Absorption and Fluorescence Emission Properties Figure 1(a) shows the UV–vis absorption spectra of pDA-based f-silica/pDA, f-FNPs, and TiO2integrated f-FNPs (f-FNPs/TiO2). An absorption band was observed at 280 nm, corresponding to integrated dopamine moieties, and an extended absorption edge was observed near 200–400 nm for f-FNPs/TiO2.5, 21 As shown in Figure 1(b), f-silica/pDA exhibited very weak photoemission, while both f-FNPs and f-FNPs/TiO2 exhibited intense fluorescence in the range of 350–700 nm with dependence on excitation wavelength, as shown in Figure 1(c) and (d). A majority of carbon dots typically exhibit excitation-wavelength-dependent fluorescence emission behavior. Hence, the carbon residues present in both f-FNPs and f-FNPs/TiO2 are believed to exhibit carbon-dot-like morphology.22 The blue emission bands under ultraviolet excitation (405 nm) significantly increased in the case of f-FNPs/TiO2, whereas the brightest emission was observed in visible light excited wavelength of 488 and 543 nm, respectively. The f-FNPs/TiO2 exhibited significantly weakened green and red emission as compared with that of f-FNPs, as shown in Figure 1(e).23 This weakened visible photoluminescence of f-FNPs/TiO2 is believed to correspond to the extremely facile transfer of electrons from f-FNPs to TiO2, which is crucial for


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minimizing the recombination of electrons and holes and enhancing the visible-light-enhanced photocatalytic activity of f-FNPs/TiO2.24 3.2. XPS Measurements The photocatalytic behavior of f-FNPs/TiO2 was attributed to the reduction of dopamine with silica-based NPs and TiO2 under alkaline condition (pH 8.5). As a result, both silica–carbon hybrid structures and TiO2 NPs are observed. Figure 2 shows the XPS spectra of f-FNPs and fFNPs/TiO2. In the Ti 2p spectrum of f-FNPs/TiO2, peaks were observed at 458.3 and 464.1 eV, characteristic of Ti 2p3/2 and Ti 2p1/2, respectively (Figure 2(b)).25 However, these peaks were not observed for f-FNPs, caused by the absence of TiO2 NPs. Characteristic XRD patterns of f-FNPs were observed in the 2θ range of 20°–40°, attributed to the hybridization of the silica and carbon lattices.26 In the figure 2(c) XRD pattern of f-FNPs/TiO2, crystallinity decreased relative to that observed in the XRD pattern of f-FNPs, possibly accounting for the increased variation in the orientation of TiO2 on the hybridized silica–carbon atoms in these NPs. Overall XPS studies demonstrated the structural integrity of f-FNPs and f-FNPs/TiO2 NPs. 3.3. Nanoparticle Size and Structure of f-FNPs/TiO2 As mentioned before, DA was used as the carbon source for synthesizing carbon spheres by self-polymerization, followed by carbonization, using a silica template. Moreover, silicon NPs have been integrated with electrochemically conductive TiO2, resulting in significant enhancement of the reversible capacity and cyclic stability of Si-based electrodes.27 Using DA and f-silane-derived f-silica/pDA composite material as the precursor source and concentrated H2SO4 as the dehydration medium, carbon-derived f-FNPs were synthesized. The successful formation of graphene-like carbonized materials was confirmed by the lattice d-spacing in the


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range of 0.390–0.410 nm (Figure 3(c)). As can be observed in the high-resolution TEM image in Figure 3(d), by the dopamine moiety, the TiO2-immobilized f-FNPs (f-FNPs/TiO2) exhibited a dspacing of 0.396–0.410 nm. As f-FNPs/TiO2 was obtained by the dopamine reduction of f-FNPs, the product possibly consists of a mixture of FNPs (silica–carbon)-integrated TiO2 as a result of post-deposition.28 Furthermore, from the TEM images, the particle size (diameter, d) of both fFNPs and f-FNPs/TiO2 were ∼200 nm. The post-deposited TiO2 on f-FNPs (silica–carbon) as demonstrated by the TEM image supported the evidence of TiO2-integrated f-FNPs (Figure 3b). 3.4. SEM Images and Confirmation of Corresponding Elemental Composition The catechol and amine functional groups containing DA biomolecules can self-polymerize, affording a pDA film under alkaline pH. This technique has been reported to form an adhesive layer for immobilizing organic and inorganic substrates.22 In addition, the use of DA or pDA as a carbon filler has been reported to demonstrate required performance for energy conversion systems.14, 28 Figure 4 shows the SEM image and the corresponding energy-dispersive X-ray (EDX) spectral patterns for the elemental composition of f-silica/pDA (Figure 4(a) and 4(a1)), fFNPs (Figure 4(b) and 4(b1)), and f-FNPs/TiO2 (Figure 4(c) and 4(c1)), respectively. The silica– carbon NPs (f-silica/pDA) and silica–carbon FNPs (f-FNPs) were oriented in the same direction, with identical size particles of 250 and 200 nm, respectively.29 On the other hand, the size of TiO2 deposited on silica–carbon FNPs (f-FNPs/TiO2) was almost the same as that observed in the TEM images stated earlier (200 nm). However, identical surface roughness is a physical indicator for the existence of TiO2 NPs. Furthermore, the composite structure was further investigated by EDX spectroscopy. As shown in Figure 4 (a1) and (b1), f-silica/pDA and f-FNPs exhibited almost the same percentage of carbon (C), nitrogen (N), oxygen (O), fluorine (F), and silicon (Si), respectively. For the TiO2 post-deposited material, a peak corresponding to titanium


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was only observed for f-FNPs/TiO2, confirming its structural identity (Figure 4(c1)).30 The results obtained from EDX were consistent with SEM images, which were further corroborated by the elemental mapping images of C, O, Ti, and Si of f-FNPs/TiO2 (Figure (4d–4g)). The particles of f-FNPs/TiO2 consist of the silica–carbon structure and TiO2 NPs as the core and shell, respectively. 3.5. Contact Angle Evaluation A potential solution for creating an antifouling substrate involves the fabrication of a nonwetting surface with the prerequisite of a very high contact angle.4 In addition, photoinduced conversion from a hydrophobic to a hydrophilic state is an interesting area of research in surface chemistry. With the discovery of amphiphilic TiO2 surfaces, the fabrication of their surface properties has already been investigated.31 For examining the photocatalytic performance of a surface coating, f-silica/pDA, f-silica/pDA/TiO2, f-FNPs, and f-FNPs/TiO2 were coated on a paper substrate, and static water contact angle measurements were conducted.32 Dynamic contact angle provides information about the roughness of solid materials via contact angle hysteresis, whereas static contact angle is more preferable for the determination of quasi-static processes. Most of the hydrophilic–hydrophobic transitions of TiO2-related solid materials are evaluated by static contact angle measurements, while static contact angles of our materials are evaluated in this study. The roughness of solid materials induces a significant effect on the wettability of solid substrates.33 As shown in Figure 5, under UV light, the contact angles of f-silica/pDA/TiO2 and fFNPs/TiO2 decreased. Moreover, f-FNPs/TiO2 also exhibited the same phenomenon, albeit under visible light. No change was observed for the control group in the absence of light (dark condition), and the superhydrophobic substrate was retained, probably attributed to the concentration of -OH groups on the TiO2 thin film surface caused by the presence of TiO2 in the


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hybrid NPs, and increases under UV irradiation, attributed to the transfer of charges between Ti ions and adsorbents such as -OH and/or O2.34 As a result, the contact angle of TiO2-containing silica–carbon NPs gradually decreases, ultimately attaining a hydrophilic state. Simultaneously, f-FNPs/TiO2 also exhibited the same behavior under visible light. The silicon and carbon nanostructure arising from the quantum-confinement effects are believed to exhibit excellent upconversion for TiO2 and harness the use of the complete spectrum of light for photocatalysis.35 3.6. Photocatalytic Kinetics in Response to UV and Visible Light The most popular topic related to wetting studies in different research areas involves the observation of the hydrophilicity and hydrophobicity of coated surfaces in an attempt to understand and reveal the mechanism involved in the penetration of a liquid into a surface. For determining the wettability of f-silica/pDA, contact angle measurements of f-silica/pDA/TiO2, fFNPs, and f-FNPs/TiO2 were conducted under time-dependent UV and visible light conditions. As shown in Figure 6(a) and 6(b), the contact angles of f-silica/pDA and f-FNPs retained hydrophobicity under both UV and visible light irradiation. Simultaneously, UV-light-treated fsilica/pDA/TiO2 exhibited decreased contact angle with time, suggesting that reactive oxygen and hydroxide species are generated from the TiO2-mediated photochemical reaction. These species increase hydrophilicity, which in turn results in the decreased contact angle. However, in the presence of visible light, only f-FNPs/TiO2 exhibited this behavior. TiO2-deposited fsilica/pDA did not exhibit any photocatalytic activity, and the contact angle remained unchanged under visible light. Figure S1(a, b) shows the reversible change from hydrophilic to hydrophobic nature via contact angle measurement under UV and visible light irradiation, respectively for 48 h and under dark conditions for 72 h. The samples stored in the dark after UV and visible light irradiation exhibited a reversible change from hydrophilic to hydrophobic nature.


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Moreover, the photocatalytic performances of f-silica/pDA, f-silica/pDA/TiO2, f-FNPs, and fFNPs/TiO2 were assessed by the degradation of the organic dye methylene blue (MB) under UV and visible light irradiation (Figure 6(c) and 6(d)).36 Under UV light, f-silica/pDA/TiO2 and fFNPs/TiO2 exhibited appreciable photocatalytic activity and good capacity to degrade MB in a time-dependent manner (Figure 6(c)). This photocatalytic activity is typically attributed to the promotion of photo-oxidation by oxidizing agents, resulting in the decrease of the absorbance of MB molecules on the TiO2 surface. However, under visible light, only f-FNPs/TiO2 exhibited photocatalytic activity (Figure 6(d)). This result indicated that solar light can be effectively utilized by TiO2, thus improving photocatalytic performance. The improvement in photocatalytic performance is possibly caused by the fact that sp2 and sp1 carbon-derived f-FNPs serve as electron acceptors and donors, respectively, resulting in the photoinduced transfer of electrons from the silicon–carbon core to the TiO2 surface and the withdrawal of excess electrons from TiO2 to the carbonized silicon–carbon surface during photochemical conversion, representing the typical pathway of up-conversion.35 Moreover, for the degradation of MB, titania-based photocatalysts induce the formation of cationic functional groups of MB molecules, followed by the initial opening of the central aromatic ring by their subsequent metabolites. Under light, the MB dye is thus oxidized and degraded on the TiO2 surface via a heterogeneous catalytic pathway.37 3.7. Antifouling Effect Photoinduced wettability demonstrates significant potential in various fields, such as antifouling, and self-cleaning, as well as for intelligent membranes. For evaluating wettability, HeLa cells were culture on the f-FNPs/TiO2-coated paper surface.32 As shown in Figure 7, cell attachment decreased as a result of the f-FNPs/TiO2 coating, which increased with increasing


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irradiation time until a time of 3 h, at which time it almost recovered back to its initial state after 60 h. These characteristics were similar to those observed under UV and visible light exposure (Figures S2 and S3, Supporting Information). As mentioned above, the f-FNPs/TiO2-modified surface exhibited hydrophobicity to inhibit the attachment of cells. However, light-irradiated photochemical conversion induced hydrophilicity, thus increasing cell attachment. Surface chemistry, as well as surface free energy, can significantly affect interactions between cells and materials. Indeed, surface characteristics affect the adsorption of proteins, which involves the interaction between the cell membrane and proteins and the mediation of the adhesion of cells to substrates.38 This property is attributed to antifouling for protecting material surfaces. The recovery of the attached cells after light irradiation from the f-FNPs/TiO2-coated paper confirmed that the sample surfaces demonstrate the spontaneous reversal of wettability, thus promoting cell attachment and detachment.

4. CONCLUSIONS In this study, carbonized silica–carbon hybrid fluorescence nanoparticles integrated with TiO2, responsible for improvement in photocatalytic activity were prepared. These fluorescent carbon hybrid nanoparticles exhibited a hydrophobic–hydrophilic transition surface. Nanosized silica– carbon core NPs (f-FNPs) were synthesized from TEOS and dopamine under mild alkaline conditions. The as-synthesized hybrid core particles were carbonized in an acidic medium, affording fluorescence silica–carbon hybrid nanoparticles, as confirmed by fluorescence emission measurements. TEM and SEM images clearly demonstrated the size and morphology of the silica–carbon core NPs and TiO2-deposited silica–carbon NPs. The elemental composition


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and structural integrity were further ascertained by XPS, EDX, and elemental mapping. The absorption of UV and visible light on the surface of f-FNPs/TiO2 caused the photo-bleaching of MB, which is represented by a change of the contact angle from hydrophobic to hydrophilic nature caused by the vigorous generation of reactive oxygen species. In addition, the anti-wetting state, which changed to a hydrophilic state after UV–vis irradiation, was also confirmed by the cell attachment study. These results indicate that a change from durable superhydrophobicity to hydrophilicity, attributed to the synergistic effect between carbon and silica and deposited TiO2, can be a foreseeable possibility in the near future.


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ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (S. Y. P). * E-mail: [email protected] (K. D. L). Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS This study was supported by Grant Nos. 10048377, 10062079, R0005303 and R0005237 from the Ministry of Trade, Industry & Energy (MOTIE), Fusion Research R&D Program from the Korea Research Council for Industrial Science & Technology (No. G02054), and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2014055946) and Marine Biotechnology Program (20150220) funded by Ministry of Oceans and Fisheries, Korea.


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Scheme 1. Schematic of visible-light-driven photocatalytic mechanism of a perfluorinated fluorescent carbon-dot-integrated TiO2 NPs surface exhibiting antifouling effects.


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Figure 1. (a) UV–vis absorption spectra of f-silica/pDA, f-FNPs, and f-FNPs/TiO2. Fluorescence emission spectra of (b) f-silica/pDA, (c) f-FNPs, and (d) f-FNPs/TiO2 at different excitation wavelengths (340–540 nm). Illuminated confocal laser scanning microscopy (CLSM) images of the aqueous solutions of (e) f-FNPs and f-FNPs/TiO2 at wavelengths of 405 nm, 488 nm, and 543 nm.


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Figure 2. X-ray photoelectron spectroscopy (XPS) survey spectra of (a) f-FNPs and (b) fFNPs/TiO2. Inset shows the Ti 2p narrow-scan spectra of (a) and (b). (c) X-ray diffraction (XRD) pattern in the 2θ range (20°–80°) of f-silica/pDA, f-FNPs, and f-FNPs/TiO2.


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Figure 3. TEM image and corresponding lattice d-spacings of (a, c) f-FNPs and (b, d) fFNPs/TiO2.


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Figure 4. SEM image and the corresponding EDX spectra for the elemental composition of (a, a1) f-silica/pDA, (b, b1) f-FNPs, and (c, c1) f-FNPs/TiO2, respectively. The corresponding elemental mapping images of the (d) C, (e) O, (f) Ti, and (g) Si elements of f-FNPs/TiO2 nanoparticles. Scale bar, 100 nm.


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Without Irradiation UV Irradiation Visible Irradiation

Contact angle (o)

140 120 100 80 60 40 20 0

2 iO /T Ps N f-F 2 iO Ps /T N f-F pDA / ca ili A f-s D /p ca ili f-s

re ba

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Figure 5. Water static contact angle (°) measurements of bare (control), f-silica/pDA, f-

silica/pDA/TiO2, f-FNPs, and f-FNPs/TiO2 coated paper surfaces in response to UV (365 nm) and visible light (>400 nm) irradiation.


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Figure 6. Water contact angle kinetics under (a) UV and (b) visible light of f-silica/pDA, f-

silica/pDA/TiO2, f-FNPs, and f-FNPs/TiO2 on the coated paper substrate. Photocatalytic kinetics under (c) UV and (d) visible light of f-silica/pDA, f-silica/pDA/TiO2, f-FNPs, and f-FNPs/TiO2 in an aqueous methylene blue (MB) solution.


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Figure 7. Study of cell attachment using HeLa cells on coated and uncoated paper. Time-

dependent variation of cell attachment on the f-FNPs/TiO2-coated paper substrate during (a) UV irradiation and (b) visible light irradiation.


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Visible-Light-Driven Photocatalysts of ... - ACS Publications

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