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Unique Dual Functions for Carbon Dots in Emulsion Preparations: Co-Stabilization and Fluorescence Probing Hua Tan, Wenxia Liu, Bei Gong, Wei Zhang, Haidong Li, Yu Dehai, Huili Wang, Guodong Li, and Lucian A Lucia Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b02755 • Publication Date (Web): 19 Aug 2015 Downloaded from http://pubs.acs.org on August 24, 2015
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Unique Dual Functions for Carbon Dots in Emulsion
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Preparations: Co-Stabilization and Fluorescence Probing
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Hua Tan a, Wenxia Liu a,b*, Bei Gong a, Wei Zhang a, Haidong Li a, Dehai Yu a, Huili Wang a, Guodong Li a, Lucian A. Lucia a,c* a
Key Laboratory of Pulp＆Paper Science and Technology (Ministry of Education), Qilu University of Technology, Jinan, Shandong 250353, China
b
Shandong Provincial Key Laboratory of Fine Chemicals, Qilu University of Technology, Jinan, Shandong 250353, China
c
North Carolina State University, Departments of Chemistry, Forest Biomaterials, Raleigh, North Carolina 27695, U.S.A. Correspondence to: W. Liu (E-mail:
[email protected]); L.A. Lucia (
[email protected])
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Recently, carbon dots (CDs) have drawn much attention as evidenced by their incorporation in
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many branches of science and engineering. Herein, a further unique application is elucidated:
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CDs that are synthesized by hydrothermal treatment of gelatin for a dual functionality as
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expressed in co-stabilization of particle-based emulsions and their concomitant role as
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fluorescent probes. CDs either with or without gelatin matrixes induce aggregation of Laponite
20
particles. The introduction of CDs thus enhanced the stability of Laponite-stabilized emulsions
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and promoted the formation of multiple emulsions and emulsions with fine and uniform droplets
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when the CDs-to-Laponite mass ratio was less than 45% and exceeded 60%, respectively.
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However, CDs without gelatin matrixes show slightly higher efficiency than CDs within gelatin
24
matrixes for the co-stabilization of emulsions. CDs also co-stabilized emulsions with Laponite to
25
allow the distribution of Laponite particles to be traced and the emulsion profiled under UV.
26
Keywords: Carbon dots; emulsion; photoluminescence; surface activity; Laponite; stability
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1. INTRODUCTION
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Carbon dots (CDs) or carbon quantum dots are a class of carbon-based nanoparticles with
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dimensions less than 10 nm.1,2 Fundamentally, they are sp2-hybridized graphite nanocrystals.
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CDs composed of a few atomic layers are nominally graphene quantum dots,1 while those made
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from polymers (including proteins) are polymer dots.2 Because of diverse fabrication and
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functionalization/modification methods, CDs may also contain oxygen, hydrogen, nitrogen, and
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various functional groups or alkyl ligands.3 As a class of novel fluorescent materials, CDs feature
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many advantages over conventional semiconductor quantum dots such as high water solubility,
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excellent biocompatibility, cell membrane permeability, good photo-stability, low cost,
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abundance, uniform particle size, easy modification/functionalization, and high robust
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near-infrared to near-UV luminescence.1,2,4-6 These features have aroused intense interest within
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various branches of science,7 especially in work related to bioimaging, biosensors,
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biomolecule/drug
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catalysis/electrocatalysis/photocatalysis,6,13,14 detection of substances,15 fabrication of solar cells
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and supercapacitors,16,17 electron donors and acceptors or sunlight collectors, and light-emitting
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device luminescent materials.18
delivery
fluorescent
materials,7-12
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By reducing chloroauric acid with reduced state carbon dots (r-CDs), gold nanoparticles can be
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formed, in which the r-CDs act as a reductant for Au(III) and capping agent/stabilizer to prevent
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nanoparticles from aggregation.19 However, the application of CDs as a fluorescent
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stabilizer/co-stabilizer for emulsions has not been reported to date. 2 ACS Paragon Plus Environment
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An emulsion is a stable mixture of two liquids that are normally immiscible (e.g., water and oil),
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generally stabilized by surfactants and polymers and found in many industries such as
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papermaking, food, pharmaceuticals, and cosmetics. Currently, many efforts focus on stabilizing
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emulsions using fine particles as an alternative to conventional surfactants because particles can
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avoid the adverse effects of surfactants (such as foaming by air entrapment and potential
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environmental and human health side-effects) while also providing emulsions with high
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coalescence stability and a high discontinuous phase content.20-22 Particle-stabilized emulsions
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are known as Pickering emulsions that are characterized as particle-stabilized emulsions.
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Particle localization at the oil-water interface forms a mechanical barrier around the emulsion
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droplets and prevents coalescence; therefore, the particles must be wetted by both water and oil.
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CDs are expected to perform well in this latter regard because they are at their core carbonaceous
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nanoparticles that possess hydrophilic functional groups.
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Laponite is a synthetic hectorite with disk-shaped crystals that has been extensively used as an
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emulsion particle stabilizer after modification with surfactants23 or small surfactant-like
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molecules24-29 such as tetramethylammonium chloride,24 melamine,25 short-chain aliphatic
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amines,25,29 alanine,27 and urea.28 However, surfactants may pose potential biological threats and
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foaming problems,30 while small surfactant-like molecules can only minimally reduce the
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hydrophilicity of Laponite. With respect to physical interrogation, both surfactants and small
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surfactant-like
molecules
demonstrate
no
fluorescence
and
thus,
the
imaging
of 3
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Laponite-stabilized emulsions must be done with fluorescence dyes.
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In this study, we report CDs prepared by hydrothermal carbonization of gelatin for a first time
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use as a fluorescent modifier to adjust the wettability of Laponite particles and as a fluorescent
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particle stabilizer to co-stabilize emulsions with Laponite. The oil is liquid paraffin that has been
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extensively employed in commercial emulsion studies such as papermaking, cosmetics,
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petrochemicals, and oil refining.
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Gelatin was selected as the precursor to prepare CDs due to its abundant amino and carboxyl
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groups. It is a water-soluble polymer composed of peptides and proteins produced by partial
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hydrolysis of collagen from animal skin and bones. It has long been used in food,
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pharmaceuticals, and cosmetic manufacturing; moreover, the carboxyl and amino groups tend to
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condense, dehydrate, and form cross-linked and carbonized structures upon heating without the
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assistance of any chemicals in pure water, whereas the polymer chains, which stretch from the
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cross-linked and carbonized center, can passivate and stabilize the carbonized center. Thus, the
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CDs were prepared using a one-pot hydrothermal method at relatively low temperatures.2,31
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Meanwhile, the use of low temperature was beneficial to obtain both well-passivated and
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functionalized CDs. The properties of the carbon dots and their effect on a Laponite aqueous
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dispersion and paraffin/water emulsions have been the subject of investigation in the current
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study. The as-prepared CDs either with or without gelatin matrixes possessed carboxyl and
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amino groups that were able to interact with Laponite leading to the reduction of negative ζ 4 ACS Paragon Plus Environment
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(zeta) potential and inducing the aggregation of Laponite particles, whereas the fluorescence of
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CDs was not significantly reduced by interaction with Laponite. Consequently, CDs
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fluorescently mark Laponite particles and improve the stability of Laponite stabilized emulsions.
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Because of their excellent biocompatibility, low cost, and easy preparation, CDs co-stabilize
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emulsions made with nanoparticles and thus may find applications in numerous fields.
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2. EXPERIMENTAL SECTION
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Materials. Gelatin was purchased from Tianda Chemical Reagents Factory (Dongli district,
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Tianjin, China), and used without further purification. Liquid paraffin with a purity greater than
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99% (d204=0.835–0.855) was provided by Damao Chemical Reagent Company (Tianjin, China).
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Laponite was a product of Rockwood Additives Ltd. (UK) supplied as a white powder named
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Laponite RD. The white powder was composed of disk-shaped crystals with a thickness of ~ 1
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nm and a diameter of 25-30 nm. Deionized water, which was prepared by ion exchange, was
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used for chemical dissolution and particle dispersion.
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Preparation and characterization of fluorescent CDs. CDs were synthesized by dissolution of
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gelatin in warm water followed by hydrothermal treatment at 200 °C for 3 h according to Liang
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et al.31 The complete dissolution of gelatin required pre-swelling of gelatin at room temperature
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for 72 h. The detailed process and characterization of fluorescent CDs may be found in the
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accompanying Supporting Information section. The CDs prepared by gelatin without and with
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pre-swelling are respectively labeled as CDs-1 and CDs-2. 5 ACS Paragon Plus Environment
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Effects of CDs on properties of Laponite aqueous dispersion. A Laponite aqueous dispersion
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was prepared by dispersing Laponite in deionized water until it swelled completely.28,32,33 A CD
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aqueous dispersion was directly added to Laponite aqueous dispersion at a pre-determined mass
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ratio of CDs to Laponite under agitation. A fluorospectrophotometer (HITACHI F-4500, Hitachi
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Co., Japan) probed its fluorescence properties. The interaction of CDs with Laponite was
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detected by IR spectroscopy and XRD after being dried at 100 °C. The surface and interface
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tension between paraffin and the aqueous dispersion of CD-Laponite were detected with a Krüss
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K100MK2 tensiometer (KRÜSS GmbH, Germany) at 25 °C.
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Preparation and characterization of emulsions co-stabilized by Laponite and CDs.
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Emulsions were prepared at 25 °C by adding paraffin oil into the CDs-Laponite aqueous
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dispersion, and homogenizing the mixture using a FM200 high shear emulsifier (FLUKO
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Equipment Shanghai Co., Ltd.) with a 1.0 cm head at 6000 rpm for 3 min. The as-prepared
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emulsions were then transferred into glass vessels for observation of emulsion stability. The
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stabilities of the emulsions relative to creaming and coalescence were assessed by monitoring the
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release of water and oil, respectively after the emulsions were prepared for 24 h.34-36 The
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emulsion stabilities were then evaluated by the emulsion volume fraction, which is defined as the
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ratio of emulsion volume to the total volume of emulsified system.26 To observe the morphology
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and distribution of CDs on emulsion droplet surfaces, the emulsions were imaged by both a laser
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confocal microscope (FV300-LX71, Olympus Corporation, Japan) and a Rise-3002 optical 6 ACS Paragon Plus Environment
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biomicroscope (Jinan Runzhi Science and Technology Co., Ltd.).
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3. RESULTS AND DISCUSSION
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Characterization and optical properties of as-prepared CDs. TEM images (Figure S1a and b,
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Supporting Information) show that the as-prepared CDs-1 possess a nearly spherical shape and
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space irregularly, but rather densely, within gelatin matrixes, which originate from unreacted
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gelatin precursors and are much larger than the CDs. However, the as-prepared CDs-2 does not
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contain any gelatin matrixes due to full pre-swelling before dissolution and hydrothermal
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treatment (Figure S1c, Supporting Information). The CDs, including both CDs-1 and CDs-2, are
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almost mono-dispersed with sizes ranging from 5 nm to 10 nm. The HR-TEM image of a single
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CD from CDs-1 (inset of Figure S1b, Supporting Information) and CDs-2 (Figure S1d,
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Supporting Information) indicates that the two CDs have the same ordered lattices.
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The XRD patterns (Figure S2a and S2b, Supporting Information) further reveals that the
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interlayer spacing of (002) plane of the CDs-1 and CDs-2 are 0.42 nm and 0.43 nm both of
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which are larger than that of graphite (0.34 nm)31,37 and the average molecular distance of gelatin
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supporting the existence of abundant functional groups during the formation of carbon dots.31
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The larger (002) spacing (d(002)) for CDs-2 is probably ascribed to the removal of gelatin
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matrixes.
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By comparing the FT-IR spectra of CDs-1 and CDs-2 with that of gelatin and analysis for the
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absorption bands of CDs and gelatin (Figure S2c and S2d, Supporting Information), it was found
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that alkyl and aryl groups as well as cumulated alkenes are formed in the as-synthesized CDs.
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However, amino-containing functional groups and carboxylic acid functional groups can still be
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found on both the CDs-1 and CDs-2, revealing the amphoteric characteristics of CDs inherited
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from gelatin.31 Analysis of XPS spectra (Figure S3, Table S1 and Table S2, Supporting
159
Information) further confirm the inclusion of carbonization/condensation in the preparation of
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the CDs originating from the functional groups of C-O and O=C-NH that then form C-C, and
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N-O in addition to the presence of amino and carboxyl groups.
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The UV-Vis absorption spectra of pristine gelatin and the CDs-1 (Figure S4a, Supporting
164
Information) also indicate that carbonization/condensation reactions among the functional groups
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of gelatin lead to the formation of a C=C bond. The as-prepared CDs-1 emit a blue-purple glow
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(inset in Figure S4a, Supporting Information) under UV light having a peak wavelength of 365
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nm in aqueous medium and is representative of the general optical properties of CDs, such as the
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variations of their emission peak in both intensity and emission wavelength with an increase in
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excitation wavelength31,38-40 and concentration
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The maximum excitation and emission wavelengths of the synthetized CDs-1 are at 365 nm and
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416 nm, respectively (Figure S4c, Supporting Information), while the maximum fluorescence
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intensity of the CDs occurs at a concentration of 11.7 g/L (Figure 4d, Supporting Information).
41
(Figure S4b and 4d, Supporting Information).
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Interaction of CDs with Laponite and effect of CDs on interfacial tension between liquid
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paraffin and water. Laponite is a magnesium silicate crystal with negatively charged crystal
176
faces due to the isomorphic substitutions of Mg2+ with Li+. It swells in water and forms a clear
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and colorless colloidal dispersion. The as-synthesized CDs carry both positive and negative
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charges after their amino and carboxyl groups are ionized in aqueous medium while their net
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surface charge is negative as judged by their negativeζpotential (Figure S5, Supporting
180
Information) due to the weak alkaline pH of the CD aqueous dispersion (~ pH 8). Therefore, they
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are expected to interact modestly with the Laponite in aqueous medium. Figure 1 shows the IR
182
spectra and XRD patterns of CDs-1-Laponite and CDs-2-Laponite with various CDs-to-Laponite
183
mass ratios. As shown in Figure 1a and c, the IR spectrum of pristine Laponite possesses
184
absorption peaks at 1004 cm-1 and 663 cm-1 that can be attributed to the stretching vibration of
185
Si-O and Mg (Li)-O (OH) groups, respectively. The absorption peaks at 3600-3100 cm-1 (3445
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cm-1) and 1630 cm-1 are correlated to the O-H groups from hydrogen-bonded silanol groups and
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hydration water, whereas the shoulder band at 3687cm-1 can be assigned to the stretching
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vibration of Mg-OH.24,25,27,28 In the IR spectra of CDs-1-Laponite and CDs-2-Laponite with
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different CDs-to-Laponite mass ratios, in addition to the absorption bands that belong to
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Laponite and CDs, a new absorption band at 3712 cm-1 appears which may originate from the
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stretching vibration of vaporized water.42 Meanwhile, the shoulder band at 3687 cm-1, which
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originates from the Mg-OH stretching vibration of Laponite, is more separated from the other
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absorption bands of O-H stretching vibrations with an increase in the CDs-to-Laponite mass ratio
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due to the red shift of the latter. In addition, the absorption band correlated to the other O-H 9 ACS Paragon Plus Environment
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stretching vibrations becomes stronger. This indicates that the CDs adsorb on the Laponite
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particles and replace the adsorbed water for hydrogen bonding with silanol groups because the
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absorption band of O-H stretching vibration of the CDs appears at 3293 cm-1 (Figure S2,
198
Supporting Information), which does not overlap as much with the stretching vibration of
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Mg-OH versus that of adsorbed water. Since the functional groups of CDs inherited from gelatin,
200
the IR spectra of CDs-1-Laponite and CDs-2-Laponite are quite similar.
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202 203
Figure 1. Infrared spectra and XRD patterns of (a, b) CDs-1-Laponite and (c, d) CDs-2-Laponite
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at different CDs-to-Laponite mass ratios. The “CDs/Laponite” in the figures represents the
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CDs-to-Laponite mass ratio.
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As shown in Figure 1b, a very broad diffraction band centered at 2θ= 6.96° appears in the XRD
208
pattern of pristine Laponite that originates from the diffraction of (001) plane, corresponding to a
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interlayer distance (d(001)) of 12.69 Å. After the introduction and increase in the CDs-to-Laponite
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mass ratio, the diffraction bands of both CDs-1-Laponite and CDs-2-Lapontie shift towards
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smaller angles, and the intensity of the diffraction bands become stronger, which means that both
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the d(001) (also shown in Table S3 and S4, Supporting Information) and particle size of the two
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CDs-Laponite are increased by CDs. This indicates that the two CDs are able to interact with
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Laponite either by hydrogen bonding or by dipole-dipole interactions. However, the equilibrium
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d(001) of CDs-1-Laponite and CDs-2-Laponite are only 1.45 nm and 1.47 nm, respectively,
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suggesting that the adsorbed CDs on Laponite surfaces occur as individual platelets. The larger
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d(001) of CDs-2-Laponite than that of CDs-1-Laponite is attributed to the larger thickness of
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CDs-2, which can be judged by the larger d(002) of CDs-2, than that of CDs-1 platelets.
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Figure 2 shows the effect of the CDs-to-Laponite mass ratio on the conductivity, viscosity, pH,
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and turbidity of CDs-1-Laponite and CDs-2-Laponite aqueous dispersions as well as the ζ
222
potential of Laponite. The insets show the appearances of CDs-Laponite aqueous dispersions
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with various CDs-to-Laponite mass ratios. It can be found from the Figure 2a and c that the
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conductivity of the two CDs-Laponite aqueous dispersions are linearly increased by increasing
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the CDs-to-Laponite mass ratio, implying that both the CDs-1 and the CDs-2 are
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electroconductive from the ionization of their functional groups. The interaction of CDs-1 and
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CDs-2 with Laponite particles increases the apparent viscosity of the Laponite aqueous 11 ACS Paragon Plus Environment
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dispersion because the interaction induces aggregation of Laponite particles as shown in the
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insets in Figure 2b and 2d. The CDs-1-Laponite dispersion and CDs-2-Laponite aqueous
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dispersion have very similar apparent viscosities and reach their maximum apparent viscosities at
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60% of CDs-to-Laponite mass ratio, suggesting that the interaction of CDs-2 with Laponite
232
particles is very similar to that of CDs-1 because the functional groups of the CDs are both
233
inherited from gelatin. The slight difference is that CDs-2-Laponite aqueous dispersion displays
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a slightly lower viscosity due to its smaller apparent particle size from the absence of gelatin.
235
When the CDs-to-Laponite mass ratio is higher than 60%, the apparent viscosities of the two
236
CDs-Laponite aqueous dispersions decrease probably due to the dehydration of Laponite
237
particles.
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Figure 2. Variation of (a, c) conductivity and apparent viscosity; (b, d) pH and turbidity of
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CDs-1-Laponite (a, b) and CDs-2-Laponite (c, d) aqueous dispersion as well as (b, d) ζ 12 ACS Paragon Plus Environment
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potential of Laponite with CDs-to-Laponite mass ratio. The insets in band d are the photographs
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of CDs-1-Laponite and CDs-2-Laponite aqueous dispersions with CDs-to-Laponite mass ratios
243
of 0, 15%, 30%, 45%, 60%, 75%, 90%, and 150%, respectively, from left to right. The
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concentration of Laponite is fixed at 5 g/L.
245 246
Pristine Laponite has a negativeζpotential of more than 50 mV in an 5 g/L aqueous dispersion,
247
while its aqueous dispersion has a pH of about 9.8-10, a turbidity of less than 10 NTU. The
248
introduction of either CDs-1 or CDs-2 significantly reduce the negativeζpotential of the
249
Laponite when the CDs-to-Laponite mass ratio is lower than 30% (Figure 2b), thus providing
250
direct evidence for the interaction of CDs with Laponite. After the CDs-to-Laponite mass ratio
251
exceeds 30%, the reduction of the negativeζpotential becomes much lower because the CDs
252
carry net negative charges (Figure S5, Supporting Information). With the addition of CDs and
253
increase in CDs-to-Laponite mass ratio, the clear Laponite dispersion becomes cloudy due to
254
aggregation of Laponite particles induced by the interaction of CDs with Laponite particles,
255
whereas the turbidity increases exponentially when the CDs-to-Laponite mass ratio is less than
256
75%. At a CDs-to-Laponite mass ratio of 75%, the turbidities of CDs-1-Laponite and
257
CDs-2-Laponite aqueous dispersions are as high as 128.4 and 118.0 NTU, respectively, and do
258
not change significantly with an increase in the CDs-to-Laponite mass ratio. The slightly lower
259
turbidity of CDs-2-Laponite aqueous dispersion is ascribed to the absence of larger gelatin
260
matrixes. The pH of CDs-Laponite aqueous dispersion is continuously decreased suggesting that
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the interaction of CDs with Laponite can prevent release of hydroxide anions from Laponite 13 ACS Paragon Plus Environment
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particles. When the CDs-to-Laponite mass ratio reaches 150%, the pH of CDs-1-Laponite and
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CDs-2-Laponite aqueous dispersions dropped to ~ 8, very close to pH (~ pH 8.0) of CDs-1 and
264
CDs-2 aqueous dispersions.
265 266
A decrease in interfacial tension plays an important role in emulsion preparation when using low
267
molar mass surfactants and surface-active polymers as stabilizers. Even for particle stabilizers, a
268
variation of interfacial tension may affect the emulsion stability and morphology.24,25,27,28
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Therefore, the effect of CDs on water surface tension and interfacial tension between paraffin
270
liquid and water was investigated. The results are shown in Figure 3.
271 272
Figure 3. Effects of CDs-to-Laponite mass ratio on (a) surface tension of CDs-1-Laponite
273
aqueous dispersion and interfacial tension between liquid paraffin and CDs-1-Laponite aqueous
274
dispersion, (b) surface tension of CDs-2-Laponite aqueous dispersion and interfacial tension
275
between liquid paraffin and CDs-2-Laponite aqueous dispersion. The Laponite concentration in
276
the aqueous dispersion is 10 g/L. The surface tension of CDs-1/CDs-2 aqueous dispersion and
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the interfacial tension between liquid paraffin and CDs-1/ CDs-2 aqueous solution versus
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CDs-to-Laponite mass ratio at the same CD concentration were also plotted for the purposes of 14 ACS Paragon Plus Environment
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comparison.
280 281
As shown in Figure 3a, both the water surface tension and the paraffin-water interfacial tension
282
are significantly reduced by the introduction of the CDs-1, further substantiating the
283
surface-active properties of the as-synthesized CDs. The surface activity of the CDs is ascribed
284
to the occurrence of gelatin matrixes because gelatin is surface active (Figure S6, Supporting
285
Information). However, the surface activity persists after the gelatin matrixes is removed by
286
pre-swelling during the preparation of CDs (Figure 3b). Furthermore, by comparing Figure 3a,
287
3b and Figure S6, it can be found that the CDs-2 show slightly higher surface activity than
288
CDs-1, while the CDs-1 shows slightly higher surface activity than gelatin. This suggests that the
289
surface activity of the CDs, especially the CDs-2 may also be attributed to carbonization and the
290
introduction of hydrophilic functional groups. When Laponite is in the aqueous dispersion, the
291
CDs lose their surface activity. Both the surface tension of CDs-Laponite aqueous dispersion and
292
the interfacial tension between paraffin and CDs-Laponite aqueous dispersion hardly change by
293
introduction of CDs and the increase of CDs-to-Laponite mass ratio. This is similar to the
294
behavior of gelatin (Figure S6, Supporting Information) and low concentration Span® 80 at the
295
paraffin-water interface when silica is present,43 indicating strong interactions between CDs and
296
Laponite particles. As a consequence, most of the CDs adsorb onto Laponite particles, while the
297
free CDs, if there are any, are insufficient to lower the water surface tension and paraffin-water
298
interfacial tension.
299
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300
Preparation of emulsions stabilized by CDs-Laponite. Although the as-prepared CDs,
301
including those with and without gelatin matrixes, are surface active and their precursor gelatin
302
can act as a stabilizer for paraffin-in-water emulsion (Figure S7a, Supporting Information), they
303
cannot provide adequate stability for emulsions when used as the lone stabilizer (Figure S7b and
304
c, Supporting Information). Both oil and water are quickly released after emulsification even
305
when the concentration of CDs is as high as 11.7 g/L, and there is hardly an emulsion phase after
306
the emulsion has been prepared for 24 h. This is likely because their surface activity is lower
307
than needed or their particles are too small to form a competent interfacial film around the
308
emulsion droplets because the stabilization energy of a particle stabilized emulsion is directly
309
proportional to the square of particle size.20 However, by the introduction of Laponite particles,
310
the emulsion becomes stable to coalescence, i.e., no oil is released when the Laponite
311
concentration is higher than 5 g/L. Therefore, emulsions co-stabilized by CDs and Laponite of
312
different CDs-to-Laponite mass ratios can be prepared by homogenizing a mixture of equal
313
volumes of paraffin oil and CDs-Laponite aqueous dispersion at a fixed Laponite concentration
314
of 5 g/L.
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315 316
Figure 4. Emulsion volume fraction of emulsions co-stabilized by Laponite with (a) CDs-1, (b)
317
CDs-2 as a function of CDs-to-Laponite mass ratio after being prepared for 24 h; (c) appearance
318
of gelatin stabilized emulsions after being prepared for 0 h and 24 h; average droplet size of
319
as-prepared emulsions co-stabilized by Laponite with (d) CDs-1, (e) CDs-2 and (f) gelatin as a
320
function of CDs/gelatin-to-Laponite mass ratio. The inset in (a, b) shows the appearance of
321
corresponding emulsions co-stabilized by CDs and Laponite. The insets in (d, e, f) are the optical
322
microscope images of as-prepared emulsions, in which the scale bar is 50 µm. The
323
CDs-to-Laponite mass ratio are 0, 15%, 30%, 45%, 60%, 75%, 90% and 150% in sequence. The
324
concentration of Laponite is fixed at 5 g/L. The paraffin to water volume ratio is 1:1 in all cases.
325 326
Figure 4a and b shows the stability of emulsions stabilized by Laponite together with CDs-1 and
327
CDs-2, respectively. The emulsion stability is characterized by emulsion volume fraction after
328
the emulsion has been prepared for 24 h. The insets in Figure 4a and b are the appearance of the 17 ACS Paragon Plus Environment
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329
corresponding emulsions with CDs-to-Laponite mass ratios from 0 to 150%. It can be seen that
330
all the emulsions are milky-white that do not evidence release of oil (insets), and are identified as
331
oil-in-water species by conductivity measurements (Figure S8, Supporting Information).
332
However, there is water released when the CDs-to-Laponite mass ratio is less than 30% for an
333
emulsion stabilized by Laponite and CDs-1. When the CDs-to-Laponite mass ratio reaches and
334
exceeds 30%, all the emulsions, including those stabilized by Laponite with CDs-2, have an
335
emulsion volume fraction of 100%, suggesting that the two CDs can improve the creaming
336
stability of Laponite-stabilized emulsions as a co-stabilizer due to their interaction with Laponite
337
particles. The interaction of CDs with Laponite induces the aggregation of Laponite particles
338
(Insets in Figure 2b and d), and improves the oil-wettability of Laponite (Figure S10, Supporting
339
Information), both phenomena which favor adsorption of Laponite particles at oil-water interface.
340
Compared Figure 4a and b with Figure 4c, the appearance of emulsions co-stabilized by gelatin
341
and Laponite, it can be found that the CDs show higher efficiency than gelatin in co-stabilizing
342
the emulsion with Laponite. As a consequence, the removal of gelatin matrixes favors the
343
preparation of emulsions with a higher emulsion volume fraction at low CDs-to-Laponite mass
344
ratio, e.g., emulsion with 100% emulsion volume fraction prepared at a CDs-2-to Laponite mass
345
ratio of 15%.
346 347
Figures 4d and 4e show the morphology (inset) and average droplet size of emulsions
348
co-stabilized by Laponite with CDs-1 and CDs-2, respectively, with various CDs-to-Laponite
349
mass ratios. Apparently, all the as-prepared emulsions either stabilized by Laponite particles or 18 ACS Paragon Plus Environment
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350
co-stabilized by CDs and Laponite particles possess spherical droplets (insets). However, the
351
droplet size of Laponite stabilized emulsion is increased due to the formation of multiple
352
emulsions by the introduction of either CDs-1 or CDs-2 at low CDs-to-Laponite mass ratios, and
353
multiple W/O/W emulsions having the largest apparent droplet diameter are observed at a
354
CDs-to-Laponite mass ratio of 30%. The emulsion droplet size decreases with an increase in the
355
CDs-to-Laponite mass ratio when the CDs-to-Laponite mass ratio is higher than 30% indicating
356
that the formation of CDs-Laponite composite aggregates helps prevent the coalescence of
357
emulsion droplets. When the CDs-to-Laponite mass ratio is higher than 45%, the droplet
358
diameter of emulsions co-stabilized by CDs and Laponite is smaller than that stabilized only by
359
Laponite and the droplets become more uniform (Figure S9a and b, Supporting Information).
360
The emulsion with the smallest droplet size and most uniform droplets is prepared at 75% of
361
CDs-to-Laponite mass ratio, at which the CDs-Laponite aqueous dispersions reach their
362
maximum turbidity (Figure 2b and d), i.e., the CDs induce a complete aggregation between CDs
363
and Laponite particles. Compared Figure 4d and e with Figure 4f, the morphology (inset) and
364
average droplet size of emulsions co-stabilized by Laponite and gelatin, it can be seen that the
365
average droplet sizes of the two CDs co-stabilized emulsions are almost same and display similar
366
variations with that of gelatin co-stabilized emulsion due to the similarity of their functional
367
groups with the that of gelatin. However, the two CDs exert much stronger effect on the average
368
emulsion droplet size than gelatin. More importantly, the gelatin cannot induce formation of a
369
multi-phase emulsion. This further demonstrates that the function of CDs in co-stabilizing
370
emulsions is dependent more on the properties of CDs than the gelatin. 19 ACS Paragon Plus Environment
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371 372
The formation of a multi-phase W/O/W emulsion is ascribed to the unevenness of the
373
CDs-Laponite composite in both wettability and particle shape when the CDs-to-Laponite mass
374
ratio is less than 45%, resulting in a large contact angle hysteresis. Consequently, the
375
CDs-Laponite composites can stabilize both inner and outer droplets of the multiple emulsions.44
376 377
The unevenness of the CDs-Laponite composite can be ascertained by the difference of Laponite
378
and CDs in both composition and shape, as well as the incomplete aggregation between CDs and
379
Laponite particles as the CDs-to-Laponite mass ratio is lower than 45% (Figure 2b and 2d). The
380
Laponite particles adsorbed with CDs, i.e., CDs-Laponite composite aggregates, have a higher
381
oil-wettability than unmodified Laponite particles (Figure S10, Supporting Information), leading
382
to wettability heterogeneities by a partly coagulated CDs-Laponite composite.
383 384
Photoluminescence imaging of emulsion co-stabilized by CDs and Laponite particles. Figure
385
5a shows the PL spectra of CDs-1-Laponite aqueous dispersions with various CDs-to-Laponite
386
mass ratios. As shown in Figure 5a, the PL spectrum of the CDs-1 in aqueous dispersion is
387
hardly affected by the introduction of Laponite. With a decrease in the CDs-to-Laponite mass
388
ratio, the fluorescence intensity of the CDs-Laponite aqueous dispersions decreases due to the
389
decrease in the concentration of CDs, especially when the CDs-to-Laponite mass ratio is lower
390
than 220%. However, fluorescence intensity is still rather high at the CDs-to-Laponite mass ratio
391
of 100%. Therefore, a laser-induced confocal scanning microscope was used to image emulsions 20 ACS Paragon Plus Environment
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with moderate CDs-to-Laponite mass ratios.
393 394
Figure 5. (a) PL spectra of CDs-1-Laponite aqueous dispersion with various CDs-to-Laponite
395
mass ratios, the excitation wavelength was fixed at 365 nm; (b) confocal fluorescence and
396
corresponding optical microscope images of emulsion stabilized by CDs-1-Laponite with a
397
CDs-to-Laponite mass ratio of 60% immediately after preparation. The liquid paraffin to water
398
volume ratio of emulsion is 1:1. The Laponite concentration is 5 g/L. The scale bar = 10 μm.
399 400
From Figure 5b, the confocal fluorescence microscope image of emulsion stabilized by
401
CDs-1-Laponite with a CDs-to-Laponite mass ratio of 60%, it can be seen that the green
402
fluorescence emitted from the CDs-1 under UV excitation (488 nm) clearly traces the contour of
403
the emulsion droplets, indicating the competence of the CDs-1 as both a co-stabilizer of
404
emulsion and a fluorescent probe. Compared with the corresponding optical microscope images,
405
it can be determined that almost all of the fluorescent points in the laser-induced confocal
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406
microscope images are located within the emulsion droplet surfaces suggesting that the
407
CDs-1-Laponite composites provide stabilization of the emulsion by adsorbing at the surface of
408
the emulsion droplets and creating a particle barrier with negative charges around the droplets.
409
When the emulsion droplets approach each other, the negatively charged particle barrier impedes
410
the coalescence/flocculation of the droplets and enhances the stability of the emulsions.
411 412
To further evaluate the efficiency of CDs-2 as fluorescent probes and the distribution of Laponite
413
particles in multiple emulsions, emulsions stabilized by CDs-2-Laponite with various
414
CDs-to-Laponite mass ratios were imaged by confocal fluorescence microscopy. It was found
415
that all the as-prepared CDs-2 co-stabilized emulsions were fluorescently imaged except the
416
emulsion with a CDs-to-Laponite mass ratio of 15%. Figure 6 shows the fluorescence images of
417
the emulsions.
418 419
Figure 6. Confocal fluorescence images of emulsions stabilized by CDs-2-Laponite with
420
CDs-to-Laponite mass ratios of (a) 30%, (b) 45%, (c) 60%, (d) 75%, (e) 90% and (f) 150% 22 ACS Paragon Plus Environment
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421
immediately after preparation. The liquid paraffin to water volume ratio of emulsions is 1:1. The
422
Laponite concentration is 5 g/L. The scale bar = 25 µm.
423 424
Figure 6 convincingly demonstrates that the CDs-2 can fluorescently label the Laponite particles
425
when the CDs-to-Laponite mass ratio is higher than 15%, adsorb on emulsion droplet surfaces
426
together with Laponite particles, and promote the formation of stable emulsion by creating
427
particle barriers. Meanwhile, the multi-phase emulsion formed at the CDs-to-Laponite mass ratio
428
of 30% as well as the influence of the CDs-to-Laponite mass ratio on emulsion morphology can
429
be clearly traced by the fluorescence of the CDs-2. Therefore, application of CDs in
430
particle-stabilized emulsions offers a new approach to enhance emulsion stability and provide
431
insight into the stability mechanism of emulsions.
432
433
4. CONCLUSIONS
434
Two CDs with diameters ranging from 5 nm to 10 nm were successively prepared by
435
hydrothermal treatment of gelatin at 200 °C. The CDs prepared by directly dissolving gelatin are
436
scattered on gelatin matrixes and emit blue-purple fluorescence at an excitation wavelength of
437
365 nm, while the CDs prepared by pre-swelled gelatin are free CDs without gelatin matrixes.
438
As nanoparticles, the two as-prepared CDs are decorated with both carboxyl and amino groups
439
inherited from gelatin, and show surface activity. Although the two CDs cannot provide adequate
440
stability for emulsions when used as the lone stabilizers, they can improve emulsion stability by 23 ACS Paragon Plus Environment
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441
interacting with Laponite particles when used to stabilize an emulsion with Laponite particles.
442
The CDs without gelatin matrixes show higher co-stabilization efficiency than the CDs with
443
gelatin matrixes. The interaction of the CDs with Laponite particles induced the formation of
444
W/O/W multiple emulsion at a CDs-to-Laponite mass ratio less than 45%, and promoted the
445
formation of O/W emulsion with small and uniform droplet size when the CDs-to-Laponite mass
446
ratios became higher than 60%. Meanwhile, using CDs as the co-stabilizer allows the emulsion
447
to be contoured and the distribution of Laponite particles in emulsions to be traced under UV
448
excitation. This study is among the first to successfully extend the application of CDs to the
449
domain of colloid chemistry.
450 451
ASSOCIATED CONTENT
452
Supporting Information
453
Additional figures, tables and corresponding discussions: TEM images of CDs; XRD patterns,
454
FT-IR and XPS spectra of CDs and gelatin; molar fractions of elements and functional groups in
455
gelatin and CDs; UV-Vis absorption spectra of CDs and gelatin; PL emission spectra of CDs; d
456
(001)
457
viscosity of CDs-Laponite stabilized emulsion as a functional of CDs-to-Laponite mass ratio;
458
surface tension of gelatin, gelatin-Laponite aqueous dispersion and interfacial tension between
459
liquid paraffin and gelatin, gelatin-Laponite aqueous dispersion; appearance of emulsions
460
stabilized by either gelatin or CDs; wettability of CDs-Laponite composites as a function of
461
CDs-to-Laponite mass ratio. This material is available free of charge via the Internet at
spacing of CDs-Laponite with various CDs-to-Laponite mass ratios; conductivity and
24 ACS Paragon Plus Environment
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Langmuir
http://pubs.acs.org.
463 464
AUTHOR INFORMATION
465
Corresponding Author
466
*W. Liu. E-mail:
[email protected]; L. Lucia: E-mail:
[email protected] 467
Notes
468
The authors declare no competing financial interest.
469 470
ACKNOWLEDGMENT
471
The project was funded by the National Natural Science Foundation of China (Grant Nos.
472
31270625, 21206086, and 21406122).
473 474
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