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Induced Aggregation of Steric Stabilizing Anionic-Rich 2-Amino-3-Chloro-5-Trifluoromethyl Pyridine on CeO QDs: Surface Charge and Electro-Osmotic Flow Analysis 2

Divya Arumugam, Mathavan Thangapandian, Archana Jeyaram, Gunadhor Singh Okram, Lalla Niranjan Prasad, and Milton Franklin Benial Amirtham J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09082 • Publication Date (Web): 27 Oct 2016 Downloaded from http://pubs.acs.org on October 28, 2016

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The Journal of Physical Chemistry

Induced Aggregation of Steric Stabilizing Anionic-Rich 2-Amino-3-Chloro-5Trifluoromethyl Pyridine on CeO2 QDs: Surface Charge and Electro-Osmotic Flow Analysis Divya Arumugam a, Mathavan Thangapandian a,*, Archana Jayaram b, Gunadhor Singh Okram c, Niranjan Prasad Lalla c, Milton Franklin Benial Amirtham a

*Author for correspondence: Dr. Mathavan Thangapandian., Ph. D., Assistant Professor, PG & Research Department of Physics, NMSSVN College, Nagamalai Madurai-625 019, Tamilnadu, India TEL: +91-9486953567, E-mail: [email protected]

a

PG & Research Department of Physics, N.M.S.S.V.N College, Madurai-625019, Tamilnadu,

India b

Department of Physics and Nanotechnology, SRM University, Kattankulathur 603203, Tamil

Nadu, India c

UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore –

452 017, India Manuscript information Total Word Count

: 8,465

Number of Text pages

: 37

Number of Figures

: 12

Table

:2

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Abstract Manufacturing quantum dots (QDs) and their suspensions through incorporation of surface coatings can have enormous attention in manipulating surface properties such as size, shape and structure. The surface of engineered QDs capped by the adsorption of organic matter is known for strongly influencing their physicochemical properties, which is very useful for many biomedical and technological applications. Prerequisite for any possible applications, the effect of organic matter on the surface of nanoparticles is important. In this contribution, owing to the privileged medicinal scaffolds based on the structure of pyridine derivatives, we have synthesized cerium oxide QDs with ~2.4 nm in size capped with anionic-rich head groups of 2amino-3-chloro-5-trifluoromethyl pyridine (ACTP), and the assay is based on the steric stabilizing capping behavior of anionic-rich ACTP molecules on the surface of CeO2 QDs. Aggregated QDs, obtained by adding capping agent in the reaction, have been characterized by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Ultraviolet visible diffuse reflectance spectroscopy (UV-Vis DRS), photoluminescence (PL) spectroscopy, transmission electron microscopy (TEM), and zeta potential techniques. XRD and FTIR analysis has been evaluated for the incorporation of ACTP on the surface of QDs. UV-Vis DRS analysis indicates the molecular aggregation predicted by dielectric confinement effect via local excitations mixed with more separated charge-transfer configurations. Under the influence of ACTP on the QDs, the electrophoretic mobility, conductivity, and current along with average electric field have been analyzed by electro-osmotic (EOS) flow analysis. In this scenario, it has been found that the addition of anionic head groups of ACTP molecules such as nitrogen atom in pyridine ring, CF3 functional groups and Cl substitution-induced steric stabilization on CeO2 2 ACS Paragon Plus Environment

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QDs (i.e. QD-to-QD interaction) has increased the ionic strength with decreasing zeta potential and promoted gradual QD aggregation. This study has thus led to a better understanding of the biophysicochemical interaction of anionic-rich ACTP molecules on the surface of CeO2 QDs and provides a physical picture of the structure and bonding of biocoated QD aggregation, which may lead to useful applications in biomedicine and designing a new material.

1. INTRODUCTION In the past decade, related to extensive applications of nanotechnology in every-day-life, unique forms of manufactured nanomaterials, nanoparticles, and their suspensions have been rapidly being created by manipulating properties such as shape, size, structure, and chemical composition using surfactants. Although the development of nanomaterials makes interesting new applications due to their inimitable properties, they have been applied extensively in a variety of areas including heterogeneous catalysts and sensor applications.1,2 Currently, due to the unique size-dependent optical and electronic properties of quantum dots (QDs), useful for versatile drug-release monitoring, efficient drug transport, and targeted cancer therapeutic applications,3-6 they have been taken to elucidate their interactions with biomolecules, cells, and tissues. One of the emblematic features of nanoparticles (NPs) is their spontaneous aggregation through noncovalent interactions such as hydrogen bonding interaction, π-π interaction, charge-



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transfer interaction, acid/base proton transfer, van der Waals forces, electrostatic forces and hostguest interaction that could have an effective mechanism in the formation of different 2D and 3D superlattices of nanocrystals.7,8 Recently, the study of aggregated molecular nanostructures have attracted considerable attention in the field of biomedical applications. Gao, et al.9 demonstrated that aggregation-induced emission NPs decorated with the Tat peptide have promising applications in tracking stem cell fate for bone repair. Lim et al.10 have shown that nearinfrared irradiation can induce phototherapeutic effects in-vitro and in-vivo of the targeted tumor cells by fabricating phthalocyanine-aggregated pluronic NPs that are capable of internalization into live cancer cells and tumor accumulation. This shows that the bindings of guest molecules into space-restricted host environments can mediate their chemical reactivity by the intrinsic properties (e.g., size, shape and chemical environment) of host materials. According to changes in these properties, QDs can swiftly agglomerate/aggregate to offer the potential for entirely new types of synthetic catalysts. Thus, the colloidal stability of host-guest surface chemistry is a function of the type of guest molecules and their environment such as surface charge, ionic strength, pH and the background electrolyte composition.11,12 Nowadays, an extensive number of guest molecules have been used as capping agents to enhance the chemical reactivity and colloidal stability of QD suspension. A wide variety of organic additives and biomolecules have been successfully used for surface-functionalization of QDs. Organic additives such as cationic,13,14 anionic15,16 and nonionic17,18 or their mixtures19 are normally used to control the size, morphology and stable dispersion of QDs. For example, Barhoum et al. found the effect of cationic and anionic surfactants on the application of calcium carbonate NPs in paper coating.20 Chen et al. synthesized alginate-coated hematite NPs in monovalent and divalent electrolytes.21 Filippo 4 ACS Paragon Plus Environment

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Gambinossi et al.22 discussed about the polymer-coated gold NPs driven by Hofmeister effects. Sohrabi et al.23 reported the molecular interactions of cationic and anionic surfactants in mixed monolayers. Zhang et al.24 investigated the interaction between five biomolecules with citratecapped gold NPs for diagnostic applications. Christensen et al.25 developed biomolecule-coated metal NPs on titanium substrate for orthopedic and biomedical applications. However, there is still a lack in understanding of the fundamental working principle of capped NPs under various physicochemical parameters such as hydrodynamic size, surface charge, ionic strength and pH within aquatic surfactant solutions. For example, which type of surfactant (cationic or anionic) and what actual mechanism take place in the environment of NPs? Notably, the mechanism involved in surface functionalization of NPs differs with capping agents since some capping agents are steric stabilizers while others are electrostatic. In contrast to the “soft repulsion” due to electrostatic stabilization in two particle interactions, steric stabilization leads to “hard repulsion” with infinite potential, higher stability and higher solid contents. As a consequence of colloidal interactions, significant changes in the particle surface charge and ionic strength are induced by varying the concentration of steric stabilizers. This in turn alters confinement effect, hydrodynamic diameter, electrophoretic mobility, conductivity and electro-osmotic (EOS) effects.26 Cerium Oxide (CeO2) NPs have been attracting great interest because of their unique applications in electrochemical devices,27 three way catalysts,28 hybrid solar cells29 and luminescent materials for violet/blue fluorescence.30 Owing to its large specific surface area and greater reactivity, these NPs are of particular interest in nanotoxicological studies, medicine, cosmetics and automotive fuel additives.



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Pyridine has been often used proton acceptor in hydrogen bonded complexes and its derivatives are of considerable interest in brain disease and stroke treatments. They act as anesthetic agents and drugs. A pyridine derivative with trifluoromethyl group is reported as a good inhibitor against the colony-stimulating factor 1 (CSF1) gene.31 Fluorine functional group in a bioactive molecule of pyridine derivative highly modifies physicochemical and biological properties of NPs due to its rich electronegativity, size, omniphobicity, lipophilicity and electrostatic interactions that can dramatically influence the chemical reactivity of organic compounds.32,33 The trifluoromethyl group-functionalized organic additives increase their chemical stability owing to the presence of C-F bond. The interaction energy utilized by fluorine participated in hydrogen-bonding is much lower than an O-H bond energy.34 Owing to the much greater bond energy of fluorinated compounds than C-H, C-Br or C-Cl bonds, they are resistant to oxidation. This will strongly influence binding affinity, bioavailability and pharmacokinetic properties of a drug, which allows it to survive in metabolic processes. Due to these properties, it is believed that ACTP exist with its vital role in medical chemistry and drug design. In the present study, we undertook the impact of steric stabilization capping behavior of ACTP molecules on CeO2 QD aggregation. The aggregation of these QDs under the influence of anionic-rich ACTP molecules in aquatic environment has been discussed in detail using Derjaguin-Landau-Verwey Overbeek (DLVO) theory. The aim of this work was to prospectively predict the aggregation of ACTP capped CeO2 QDs and clarify interaction between ACTP molecules and CeO2 QDs using various nanospectroscopy methods viz., XRD, FT-IR, Raman, PL, UV-Vis DRS, TEM, SAED and zeta potential measurements. In order to optimize the less agglomerate/aggregated media, we utilized three different aquatic media as water, ethanol and toluene for only primary CeO2 QDs. The influence of anionic-rich ACTP molecules on these 6 ACS Paragon Plus Environment

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QDs has been analyzed in optimized water medium and their dielectric confinement-induced aggregated QDs has been discussed. The adsorption of oppositely charged steric stabilizer on the surface of QDs with Ce3+ ions was attributed to decreasing zeta potential with increasing ionic strength that induces QD-to-QD aggregation (leading to increased hydrodynamic diameter). This was discussed thoroughly using various experimental techniques. Furthermore, the potential impact of ACTP has been analyzed by the direction of EOS flow; the electrophoretic mobility and conductivity have also been calculated. Our study thus made a better understanding of the biophysicochemical interaction of anionic-rich ACTP molecules on the surface of CeO2 QDs, and provides a physical picture of the structure and bonding of biocoated QD aggregation, which may lead to useful biomedical and technological applications. 2. EXPERIMENTAL SECTION 2.1 Materials. Ammonium cerium nitrate ((NH4)2Ce(NO3)6, 98% Merck), ammonia (NH3·H2O, 98% Merck), ACTP (99% Merck), ethanol, toluene and deionized water were used without further purification. 2.2 Synthesis of uncapped and ACTP capped CeO2 QDs. The three capped as well as one uncapped CeO2 QD sample synthesis was based on the use of anionic-rich biomolecule ACTP as a capping agent and (NH4)2Ce(NO3)6 and NH3·H2O as inorganic precursors. They were synthesized using simple and cost effective wet chemical procedures previously reported in the literature35,36 with slight modifications. In a typical synthesis, 0.20 g of (NH4)2Ce(NO3)6 was thoroughly dissolved in 10 ml of ethanol by stirring. Then, 0.075 mol of ammonia water was added dropwise to the precursor solution while stirring continuously for 6 h; color of the solution became pale yellow due to formation of CeO2 QD precipitate. The precipitate was separated by



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centrifugation, washed with deionized water and ethanol for several times, and dried at 80 °C for 6 h. The resulting sample was termed as CQ0. For the synthesis of ACTP capped CeO2 QDs, 0.02, 0.04, 0.06 M of ACTP were added to the solution of (NH4)2Ce(NO3)6 in NH3·H2O while similar procedure was followed as described above. The resulting samples were termed as CQ0.02, CQ0.04 and CQ0.06, respectively. 2.3 Apparatus X-ray diffraction (XRD) measurements. Powder XRD patterns were collected by using Bruker D8 Advance X-ray diffractometer in the angle range of 20°–80° with Cu Kα radiation (0.154 nm). Vibrational analysis. The functional groups present in uncapped (CQ0) and ACTP capped (CQ0.02, CQ0.04 and CQ0.06) samples were recorded by Fourier transform infrared (FTIR) spectroscopy using KBr discs over the range of 4000-400 cm-1 on a Bruker Vector 22 spectrometer. Micro-Raman spectra were recorded by using LabRam HR800 micro-Raman instrument (using 632.8 nm He–Ne laser). Optical properties measurements. The optical spectroscopy of the synthesized samples were measured by Ultraviolet-Visible diffuse reflectance spectroscopy (UV-Vis DRS) by means of a UV-Vis spectrophotometer (Cary-500, Varian Co.) using BaSO4. The primary CeO2 QDs exhibiting green emission was measured by room-temperature photoluminescence (PL) spectra at regular intervals of time by PerkinElmer LS-55 spectrophotometer. The instrument setting constant for all the samples were keep it at the scan speed of 300 nm/min. Morphological analysis. Sample images and selected area electron diffraction (SAED) patterns were recorded using TEM instrument (TECHNAI-20-G2) by drop-casting the well-sonicated solution of a few milligrams of CQ0, CQ0.02, CQ0.04 and CQ0.06 powder samples dispersed in 8 ACS Paragon Plus Environment

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5 ml of ethanol on carbon-coated TEM grids. Image J software was used to calculate particle size. Zeta potential measurements. The hydrodynamic size and surface charge of as synthesized QDs dispersions were measured using a NanoPlus-3 (Particulate system) utilizing dynamic light scattering (DLS). DLS is the most intense and frequently used technique for hydrodynamic size distribution measurements. Measurements were performed after thorough sonication of the assynthesized samples dispersed in deionized water, ethanol and toluene separated to conclude the best dispersant. Approximately 2 mg each of the samples were dispersed in 10 ml of the dispersant, for a typical run. The number of runs was in the range of 50–100 taken into account the complete dispersion of samples in aquatic media. Since deionized water was found to be best dispersant, it was used as dispersant for the capped samples. 3. RESULTS AND DISCUSSION 3.1 XRD analysis. XRD patterns of the prepared CeO2 QD samples at varying amounts of ACTP are shown in Figure 1. The as synthesized samples exhibited analogous diffraction peaks with no distinct changes in relative intensity of the peaks. All peaks in the XRD spectra were perfectly indexed as the pure cubic phase of CeO2 (Fm3m),14,15 and compared well with the standard powder diffraction (JCPDS No. 34-0394) with a lattice constant a = 5.411 Ǻ. A number of Bragg reflections corresponding to (111), (200), (220) and (311) reflections due to cubic fluorite structure of CeO2 can be seen. No extra peaks associated with impurities were observed. The highest intense peak of CQ0, CQ0.02, CQ0.04 and CQ0.06 samples corresponds to (111) plane, for the reason that fluorite structure of ceria has three low-index planes, namely {111} plane, {110} plane and {001} plane. Compared to the {111} plane, the {110} and {001} planes



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are less stable due to the higher surface energy of {110} and {001} planes. Figure 1 shows dominant {111} planes for the synthesized CQ0, CQ0.02, CQ0.04 and CQ0.06 samples, which are observed in QDs oriented along the [110] direction.37

Figure 1. XRD patterns of the pure and ACTP-capped CeO2 QD samples as noted. Inset: Schematic of (111) plane growth of quantum dots. 3.2 Vibrational analysis. Figure 2a shows the typical FTIR spectra of pure and ACTP-capped CeO2 QD samples, CQ0, CQ0.02, CQ0.04 and CQ0.06 respectively. The broad absorption in cm−1 is assigned to the existence of residual water and a hydroxyl group, corresponding to the O–H stretching frequency on the surface of the samples and the shoulder peak at around 1622 cm−1 appropriate to the bending vibration of associated H–O–H, which partly overlaps the O–C–O stretching band.38 The FTIR peaks at about 1514, 1385, 1063 and 841 cm-1 are due to the stretching frequency of Ce–O.39 The characteristic CeO2 peaks at 1514, 1063 and 841 cm-1


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shifted to lower wavelength with increasing amounts of ACTP concentration, can be attributed to C–C stretching mode, C–H in-plane bending and C–Cl stretching vibrations.40-42 The CeO2 characteristic peak at 1385 cm-1 was disappeared with increasing ACTP concentration, which

Figure 2. (a) FTIR spectra of pure and ACTP capped CeO2 QD samples. Inset: Zoomed part of main figure. (b) Raman spectra of pure and ACTP capped CeO2 QD samples (ѵ– stretching; β– in-plane bending; ρ– rocking). was ascribed to the C-N stretching vibration. Probably, a new peak was appeared at 1014 cm-1 with increasing ACTP concentration due to the CNC in-plane bending vibration. Figure 1a, inset, shows the possible shift and interaction of ACTP with CeO2 QDs. Figure 2b shows the Raman spectra of pure and ACTP-capped CeO2 QDs. The peak located at 451.88 cm-1 in CQ0 was a



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Raman active triply-degenerated F2g mode of fluorite CeO2 structure due to the symmetric breathing mode of the oxygen atoms around cerium ions.43 This peak shifts to lower wavenumber with increasing ACTP concentration, which can be attributed to CF3 rocking mode.40 These results confirmed the presence of ACTP in the CeO2 QDs and also proved that anionic-rich interaction of ACTP molecules with CeO2 fluorite structure. 3.3 Aggregation of pure and anionic-rich ACTP-capped CeO2 QDs by optical analysis. The UV-Vis DRS spectra of CQ0, CQ0.02, CQ0.04 and CQ0.06 samples are displayed in Figure 3. Their reflectance spectra were analyzed using Kubelka-Munk relation to convert the reflectance spectra into absorption spectra (Figure 3, inset). Figure 4 shows the Kubelka-Munk plots for the samples to determine their band gap energies associated with a direct and an indirect transition. The plot of [F(Rα)hv]2 versus photon energy (Figure 4, left panel) shows the direct band gap energy (Ed) values of 3.09, 3.00, 2.82 and 2.78 eV for CQ0, CQ0.02, CQ0.04 and CQ0.06, respectively. The plot of [F(Rα)hv]1/2 versus photon energy (Figure 4, right panel) shows the indirect band gap energy (Ei) values of 2.66, 2.61, 2.53 and 2.51 eV for synthesized samples. It is found that the samples show a remarkable reduction of Ed and Ei with increasing ACTP concentration. These results indicate that the adsorption of ACTP molecules could modify the surface state of CeO2 QDs at longer wavelengths and results in the red-shifting with increasing particle size due to dielectric confinement effect. Our results are consistent well with earlier reports.44,45 This dielectric confinement is due to the most probable penetration of electric force lines into the surrounding ACTP medium having a subordinate dielectric constant than that of the core CeO2 QD semiconductor. Typically semiconductor nanocrystallites surrounded by dielectrics such as organic molecules have less dielectric constant as well as refractive index. Therefore, it seems that the penetration of electric force lines emerging from CeO2 12 ACS Paragon Plus Environment

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nanocrystallite into the ACTP molecules enhances coulomb interaction, and exciton binding energy on other hand induces the dielectric confinement effect between charged particles. According to earlier literature46, inorganic cations passivated by organic cations increase the band gap with decreasing particle size due to quantum confinement effect. Our results with dielectric confinement effect, which is associated with the molecular aggregates could reveal that local excitations (Ce cations) mix with more separated charge-transfer configurations (ACTP anions), decrease the charge-transfer gap between O 2p and Ce 4f bonds of CeO2 (Ce3+ to Ce4+)44 that leads to decrease the band gap with increasing particle size. This clearly proves that the addition of steric stabilizing behavior of anionic-rich ACTP molecules induces CeO2 QD aggregation.47 Tsunekawa et al.48 and Zhang et al.49 reported that CeO2 exhibited dielectric confinement effect below the Bohr radius of 7–9 nm. From these optical studies, we suggest that the synthesized samples are QDs.

Figure 3. UV-Vis DRS spectra of pure and ACTP capped CeO2 QD samples. Inset: absorbance spectra of the samples converted from DRS spectra using Kubelka-Munk relation.



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Figure 4. Direct (left panel) and indirect (right panel) band gap energy of pure and ACTP capped CeO2 QD samples shows decreasing band gap energy values due to dielectric confinement effect. Furthermore, in order to validate the quantum nature of synthesized uncapped sample CQ0, its room-temperature PL spectrum as excitation source of 290 nm was obtained and is shown in Figure 5. The spectrum exhibited a strong green emission band at 524 nm, which can be attributed to oxygen vacancies in the CeO2 crystals having electronic energy levels below 4f band.50 Photograph of pale yellow color CQ0 sample exhibited a green emission under 295 nm UV lamp (Figure 5, inset). These results vividly confirmed that our CQ0 sample was in quantum nature with particle size ~2.4 nm (Figure 7) that is below its Bohr radius of 7–9 nm.48,49


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Figure 5. PL spectrum of uncapped CeO2 QDs, exhibited strong green emission at 524 nm. Inset: Photograph of the same sample without (left) and with (right) 295 nm UV light. 3.4 Induced aggregation of steric stabilizing capping behavior of ACTP molecules on the surface of CeO2 QDs by TEM analysis. To improve the accuracy in aggregate measurements we decided to establish a method based on TEM. TEM can also be used to characterize the primary CeO2 QDs inside the large aggregates at different molar concentrations of capping agent. Figure 6a-h shows the different magnification TEM images of synthesized samples. They indicate formation of large aggregation by anionic-rich capping behavior of ACTP molecules on the surface of CeO2 QDs at different molar concentration. The selected area electron diffraction (SAED) patterns of the corresponding images are shown in the insets of Figure 6a,c,e,g. To validate our method, the first step was used to determine the primary particle size of CeO2 QDs. Figure 6a,b shows the TEM images of CQ0 at two different magnifications. The average particle size calculated from TEM image using image J was found to be approximately 2.4 nm. The associated histogram is given in Figure 7 (inset). TEM images of CQ0.02 sample (Figure 6c,d) show that the large quantities of primary 2.4 nm CeO2 QDs aggregate loosely that is attributed to 15 ACS Paragon Plus Environment


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ACTP. When ACTP concentration increases further in CQ0.04 sample, ~2.4 nm QDs are packed more densely (Figure 6e,f). Further, CQ0.06 sample TEM images (Figure 6g,h) show that large quantities of QDs pack still more densely and in bigger size. These trends show that ACTP enables aggregation of the QDs because its anionic charges in consistent with earlier reports.51, 52 It is noted that the edge between QDs are hard to identify inside the large aggregation due to the increasing concentration of ACTP. Subsequently, we could not calculate the aggregated particle size for the samples CQ0.02, CQ0.04 and CQ0.06. These extended aggregations were due to the fact of anionic-rich interactions of ACTP molecules that induce steric stabilization on the surface of CeO2 QDs. The oppositely charged head groups of anionic-rich ACTP molecules, incorporated onto the surface of CeO2 QDs resulting in an aggregation number, could be expected to increase due to the negative charge and strong ionic bonds. These results are consistent with those of previous study53 in which various functional groups of humic acid, carboxylic ( COO−) and phenolic ( ArO−) groups with negative charges have high complexation capacity with metal ions and resulting increased ionic character. Furthermore, the amount of adsorption of oleate surfactant was much higher than cetyltrimethylammonium bromide (CTAB) on the surface of calcium carbonate particle surface due to the ability of the carboxylic group (−COO−) of oleate to bind effectively to the Ca2+ ions on the calcium carbonate particle surface via ionic bonds,20 and also from other literature, they studied about the aggregation kinetics of two surfactants such as sodium dodecyl sulfate (SDS) and CTAB and were reported that aggregation of SDS was higher than CTAB due to the anionicrich behavior of SDS.23 From these analysis it is clear that the extended aggregation of CeO2 QDs were due to the steric stabilizing anionic-rich capping behavior of ACTP molecules.


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Furthermore, in order to validate the anionic-rich nature of ACTP molecules, the electronegativity of these molecules have been calculated from the reported density functional theory (DFT) analysis. In general, fluorine (3.98 eV) and Chlorine (3.16 eV) atoms have high electronegative nature in the periodic table of elements. Likewise, Mohamed Asath et al.54 reported the Mulliken Atomic Charge Distribution and Frontier molecular orbitals (FMOs) analysis from DFT analysis. Mulliken atomic charge distribution clearly showed that the nitrogen atom in the pyridine ring, CF3 functional groups and Cl substitutions of ACTP molecule have high atomic charge values. From the FMO analysis, the electronegativity (measure of the tendency of an atom to attract a bonding pair of electrons) χ = - μ and μ is the Chemical Potential (Escaping propensity of an electron from a stable system) μ = - (IP + EA)/2 have been calculated, where IP is ionization potential (Energy needed to remove an electron from a filled orbital) = -EHOMO, EA is electron affinity (Energy released when an electron is added to an empty orbital) = -ELUMO. For ACTP, EHOMO = -6.7175 eV, ELUMO = -1.5243 eV. So the total electronegativity of ACTP molecule was χ = 4.1209 eV, clearly showing the anionic-rich nature of ACTP molecules.



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Figure 6. TEM images and SAED patterns (insets) of uncapped CQ0 (a,b), and ACTP capped CeO2 QD samples, CQ0.02 (c,d), CQ0.04 (e,f), CQ0.06 (g,h) at two different magnifications. On the basis of these results, a proposed schematic representation has been given in Figure 7, where the aggregation of CeO2 QDs under the effect of anionic-rich capping behavior of ACTP molecules are emphasized. It was observed that the addition of ACTP causes consequent formation of CeO2 QDs aggregation. As the concentration of ACTP molecules increases from 0.02 to 0.06 M, the electronegative atoms such as nitrogen in the pyridine ring, CF3 functional groups and Cl substitutions of ACTP molecules increase the ionic strength. The increased ionic strength decreases the steric self-repulsion due to the strong van der Waals attractive force.55 This local steric stabilization (destabilization) induces QD-to-QD interaction


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and they are bound together better with increasing ACTP concentration, whereas areas of intact reaction held in the particle surface chemistry between the van der Waals attractive forces and the electrostatic repulsive forces formed aggregated outer surface, and the aggregation level increases with increasing ACTP concentration upto 0.06 M for the samples in the range from CQ0.02, CQ0.04 and CQ0.06.

Figure 7. Schematic representation of anionic-rich capping behavior of ACTP molecules on the surface of CeO2 QDs. This shows that the addition of ACTP concentration increases particle aggregation by decreasing steric self-repulsion. Molecular structure of ACTP given in this representation clearly shows that the nitrogen, chlorine and fluorine atoms have high electronegative nature. The electronegativity of these atoms associated with the molecular structure of ACTP was given in the discussion part. The inset histogram is associated with the average particle size calculated from uncapped CeO2 QDs.



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3.5 Zeta potential measurements. Dry powder sample was required for TEM measurements under high vacuum conditions. For more accurate anionic-rich behavior study of ACTP on the surface chemistry, we decided to go for another measurement. The formation mechanism of NPs is strongly dependent upon the electrostatic stabilization, which was described by DLVO theory (Derjaguin and Landau, Verwey, and Overbeek).56 The overall stability of the as-synthesized system is defined by its surface charge. The surface charge is based on the aggregation tendency. Therefore, the aggregation tendency of anionic-rich nature of ACTP on the surface of QDs can be observed by surface charge analysis of zeta potential measurements. Zeta potential is defined as potential at the slipping plane, which is related to the surface charge of the particle and composition of the local chemical environment. Accordingly, we believe that zeta potential measurement would give an accurate aggregation behavior of anionic-rich ACTP molecule on the surface chemistry. In order to measure the less agglomerated particle surface in aqueous media i.e. much stable medium,

2.4 nm QDs were dispersed in three suspension media as

water, ethanol, and toluene. After optimization, the less stable aqueous medium, the influence of anionic-rich ACTP molecule on the surface of CeO2 QDs would have been analyzed. Agglomeration/aggregation of pure CeO2 QDs under the influence of ionic strength, surface charge and pH. The aggregation of CeO2 QDs due to the effects of ionic strength, pH and surface charge under different suspension media are presented in this section. Here, the average size of the CeO2 QDs were taken from over two measurements (Figure 8a-c), while those for zeta potential were averaged from more than two measurements (Figure 8d-f). The average hydrodynamic diameter of the CeO2 QDs dispersed in water medium was 204 nm higher than the primary particle size

2.4 nm, which indicates that particles are highly agglomerated.

According to DLVO theory, and the more accurate Sogami-Ise theory,57,58 the agglomeration and


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the stability of particle dispersions were determined by the sum of the attractive and repulsive forces between the individual particles. And also it was suggested that higher ionic strength of the electrolyte solution diminish interparticle repulsion due to the attractive van der Waals force, which indicated that the above hydrodynamic diameter of the CeO2 QDs dispersed in water medium was due to the dominant attractive force over the electrostatic repulsive force, resulting in a highly agglomerated particles, in which case the particle surface charge (Zeta potential) was noticed to be 39.91 mV at pH 7. Similar, agglomerated particles ( 250 nm) were observed in CeO2 QDs dispersed in an ethanol medium but slightly increasing hydrodynamic diameter, with decreasing zeta potential 36.54 mV measured at pH 6, resulting that an increase in the ionic strength owing to the compression of the electrical double layer decrease their zeta potential, and therefore promote agglomeration. The same method can be applied to determine the aggregation state of CeO2 QDs dispersed in toluene medium. Under these conditions, the average hydrodynamic size was 620.3 nm was still much higher than the TEM particle size

2.4 nm with a zeta potential of -1.39 mV

at pH 6, which indicated that the increasing ionic strength leads to increasing hydrodynamic diameter suggesting highly aggregated dispersion. Consequently, the repulsive force was weakened due to the low surface charge and the hydrodynamic diameter was increased beyond which it was measurable. Compared with the previous agglomeration state of CeO2 QDs dispersed in water and ethanol medium, the formation of cavities has a capability of overcoming the van der Waals force holding them together on agglomeration with positive zeta potential and feasible ionic strength of the dependent medium. But, in the case of toluene medium, the negative zeta potential with increasing ionic strength, enhance the particle-particle coagulation and therefore promotes particle aggregation in the liquid media.59 From the above results, it was



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observed that the strong chemical bonds (cation-anion bonding) increases ionic strength with negative surface charge leads to particle aggregation and also it was not pH dependent. Because, the pH decreases from water–to-toluene medium has not shown significant effect in the particle surface chemistry. The schematic representation of aggregation of CeO2 QDs depends upon

Figure 8. (a,b,c) The hydrodynamic size distribution curves for uncapped CeO2 QDs dispersed in water, ethanol, and toluene. The average hydrodynamic size values were calculated as 204 nm (water medium), 250 nm (ethanol medium) and 620.3 nm (toluene medium). (d,e,f) The zeta potential distribution curves for CeO2 QDs dispersed in water, ethanol, and toluene media and the average zeta potentials were calculated as 39.91 mV (water medium), 36.54 mV (ethanol medium), and -1.39 mV (toluene medium). different suspension media is given in Figure 9. Furthermore, to elucidate the stability of the nanoparticle dispersion, the mobility and conductivity of CeO2 QDs in different media also have


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been found, and given in Table 1. From these results it was concluded that the optimized stable dispersion medium was water, which one has a higher mobility and conductivity.

Figure 9. Schematic representation of agglomeration and aggregation of CeO2 QDs dispersed in different medium and indicate that water medium promotes less agglomerated particle surface.

Table 1. Mobility and conductivity of CeO2 QDs dispersed in different aquatic medium.

S. NO

CeO2 QDs dispersion in different medium

Mobility (cm²/Vs)

Conductivity (mS/cm)

1

Water

2.925e-004

0.0291

2

Ethanol

7.336e-005

0.0093

3

Toluene

-2.782e-006

0.0062



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Aggregation of steric stabilizing ACTP capped CeO2 QDs under the influence of ionic strength, surface charge and pH. Furthermore, to validate the aggregation behavior of anionicrich capping agent ACTP on the surface of CeO2 QDs, zeta potential measurements have been studied. All measurements were carried out in optimized medium, water. The average hydrodynamic diameter and the corresponding zeta potential for ACTP capped CeO2 QDs at various concentrations 0.02 M, 0.04 M, and 0.06 M for the samples are shown in Figure 10a-f. These data show that with increasing ACTP concentration from 0.02 M to 0.06 M, hydrodynamic diameter increases as 295 nm (CQ0.02) < 312 nm (CQ0.04) < 434 nm (CQ0.06) (Figure 10a-c) and absolute value of zeta potential decreases as -24.49 mV (CQ0.02) > -16.09 mV (CQ0.04) > -14.99 mV (CQ0.06) (Figure 10d-f) at circumneutral pH of 7.0. Using these measured values, it was observed that the stability of metal oxide NPs in aqueous solutions depends on a hefty coverage of ionic strength. The CeO2 QDs-to-QDs interaction decreases the repulsive EDL (electric double layer) energy of the QDs with increasing ACTP concentration. This results in the compression of EDL consequently decreasing the zeta potential and promotes particles to more aggregate. Therefore, the hydrodynamic diameter increases with decreasing zeta potential. Because classical DLVO simplifies thermodynamic surface interactions by van der Walls net attractive (−VT) or net repulsive (+VT) forces. Therefore, in our case, the negative zeta potential indicates that the attractive force is dominant over repulsive force by strong chemical bonds, which promotes particle aggregation. On the basis of these results, a schematic representation for enhanced aggregation with anionic-rich ACTP molecules has been proposed in Figure 11. This enhanced aggregation strongly suggest that the ACTP capped CeO2 QDs undergo a different aggregation mechanism in the presence of Ce ions (Ce3+). The change in zeta potential and hydrodynamic diameter


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indicates ACTP adsorption onto the surface of Ce3+ cations. This can be attributed to negatively charged atoms such as nitrogen atom in the pyridine ring, CF3 functional groups and Cl

Figure 10. (a,b,c) The hydrodynamic size distribution curves and (d,e,f) The zeta potential distribution curves for ACTP-capped CeO2 QDs. The average hydrodynamic size and zeta potential values were calculated as (295 nm, -24.49 mV), (312 nm, -16.09 mV) and (434 nm, 14.99 mV) for CQ0.02, CQ0.04 and CQ0.06 samples, respectively. substitutions of ACTP molecules adsorbed on the surface of Ce3+ cations. FTIR results also proved that the interaction of nitrogen atom in the pyridine ring, CF3 functional groups and Cl substitutions on CeO2 surface. It is known that anionic-rich layer of ACTP can interact and enter into the diffused ionic layer of Ce3+ cations through electrostatic attractive force.60 If the cluster size decreases, the amount of Ce3+ ions increases. So, when the ACTP concentration was increased, the large number of Ce3+ cations interacts strongly with electronegative charges of ACTP molecules, which can increase the ionic strength of the solution. The increasing ionic



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strength with increasing ACTP concentration would induce particle-to-particle interaction that can reduce the EDL thickness such that the net repulsive energy barrier between CeO2 QDs decreases, resulting in the increasing aggregation level on the CeO2 particle surface. Thus, aggregation of the ACTP-capped CeO2 QDs occurs through electrostatic destabilization. These results are well corroborated with the obtained results from structural and morphological characterization. Finally, we come up to the approach presented here, which can be used to understand the aggregation behavior of anionic-rich capping agent on the surface of CeO2 QDs and efficiently employed for scientist’s grasp of how to apply these nanomaterials for biomedical and technological applications such as polariton lasers or excitonic switches and so on.61, 62

Figure 11. Schematic representation of anionic-rich capping behavior of ACTP molecules measured from zeta potential analysis, showing the interaction of anionic-rich nature of ACTP molecules with positive surface charges of CeO2 QDs at varying concentration decreases zeta potential and increases hydrodynamic size.


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Electro-osmotic (EOS) flow analysis. Furthermore, to analyze the effect of ACTP on CeO2 QDs, EOS flow has also been analyzed. EOS plot of various concentration of ACTP capped CeO2 QDs were shown in Figure 12. The parameters along with EOS flow in water medium are given in Table 2. Electro osmosis is the phenomenon of liquid motion through the porous solid by an applied electric field parallel to a surface. Obviously, the EOS flow indicated the direction of these ACTP-capped CeO2 QDs translocations towards negative direction (negative direction of parabolic curve). The inner surface of CeO2 QDs, which was capped by anionic-rich groups of ACTP molecules (nitrogen atom in the pyridine ring, CF3 functional groups and Cl substitutions) was ionized with Ce3+. When dispersed in water medium, the negatively capped surface was counterbalanced by positive ions from the water medium. Under the influence of electric field along the channel, sets the EDL in motion, which screens the surface charge. The positive ions in the diffuse part of the EDL migrate towards the cathode. As a consequence of increasing ACTP concentration, the addition of anions in the EDL drags the fluid to migrate towards the anode, which results in EOS flow.63 From the direction of EOS flow analysis, it is observed that the addition of ACTP decreases zeta potential, electrophoretic mobility, conductivity, current but increases average electric field.



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Table 2. EOS flow parameters of ACTP capped CeO2 QDs in water medium. Sample

Zeta potential (mV)

Mobility (cm2/Vs)

Conductivity (mS/cm)

Avg. Electric field (V/cm)

Avg. Current (mA)

CQ0.02

-24.49

-1.909 e-004

0.0278

-84.94

-0.12

CQ0.04

-16.09

-1.254 e-004

0.0073

-84.99

-0.03

CQ0.06

-14.99

-1.169 e-004

0.0061

-85.00

-0.03

Figure 12. EOS flow plot of various concentration of ACTP capped CeO2 QDs.


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CONCLUSIONS In this paper, we demonstrated the CeO2 QDs aggregation induced by the effect of anionic-rich head groups of ACTP molecules as a capping agent. Along with detail experimental investigations, it was evident that CeO2 QDs with particle ~2.4 nm were formed. Anionic-rich head groups (nitrogen atom in the pyridine ring, CF3 functional groups and Cl substitutions) of ACTP molecules induced steric stabilization on CeO2 QDs surface (Ce3+) and gradual interaction of QD-to-QD surface formed aggregated microstructure by the addition of ACTP molecules. From UV-Vis DRS analysis, it was found that direct (3.09 > 3.00 > 2.82 > 2.78 eV) and indirect (2.66 > 2.61 > 2.53 > 2.51 eV) band gap energy values was decreased by increasing the amount of capping agent due to dielectric confinement effect. To find out the less aggregated and more stable dispersion of CeO2 QDs, the particles were dispersed in three different media such as water, ethanol, and toluene, and confirmed that water medium promoted a stable CeO2 QDs dispersion with less agglomeration. Simultaneously for ACTP capped CeO2 QDs dispersed in water medium, magnitude of surface charge decreases as 24.49 mV (CQ0.02) > 16.09 mV (CQ0.04) > 14. 99 mV (CQ0.06) but hydrodynamic diameter increases as 295 nm (CQ0.02) < 312 nm (CQ0.04) < 434 nm (CQ0.06). These dominant variations lead to a decrease in QDs stability. By means of DLVO theory, it was concluded that addition of ACTP induces dominant van der Waals attractive force over electrostatic repulsive force. This increases the solution ionic strength and promoted gradual aggregation on CeO2 QDs due to the high electronegative atoms present in the molecular structure of ACTP molecules. On the other hand, from the direction of EOS flow analysis, the EDL fashioned by ACTP capped CeO2 QDs in water medium migrated towards anode, shows that the addition ACTP decreases the absolute value of zeta potential 29 ACS Paragon Plus Environment


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(24.49 > 16.09 > 14.99 mV), electrophoretic mobility (1.909 e-004 > 1.254 e-004 > 1.169 e-004 cm2/Vs), conductivity (0.0278 > 0.0073 >0.0061 mS/cm) and current (0.12 > 0.03 > 0.03 mA). Thus, it is clear that various experimental techniques used here provide ample evidence to validate the dominant role of anionic-rich ACTP molecules on the surface of CeO2 QDs with very efficient and sensitive probe for QD aggregation, which is an important finding for biomedical and technological applications. Supporting information Details concerning apparatus and performing experiments such as XRD, vibrational analysis, optical properties measurements, morphological analysis and zeta potential measurements were used to validate the dominant role of anionic-rich ACTP molecules on the surface of CeO2 QDs. This information is available free of charge via the Internet at http://pubs.acs.org Acknowledgements We thank the centre director Dr. V. Ganesan, UGC-DAE Consortium for Scientific Research, Indore, India for allowing us to utilize freely available national facilities to university, college/ research institute user scientists, students/ professors of India as sponsored by University Grants Commission, New Delhi. Dr. Mukul Gupta and Dr. Uday Deshpande are acknowledged for XRD, FTIR and UV-Vis DRS data respectively and also we thank the Management of NMSSVN College for encouragement and permission to carry out this work. Divya Arumugam selffinanced some of the funds.


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TOC Graphic for the manuscript “Induced Aggregation of Steric Stabilizing Anionic-Rich 2-Amino-3-Chloro-5-Trifluoromethyl Pyridine on CeO2 QDs: Surface Charge and ElectroOsmotic Flow Analysis” Divya Arumugam a, Mathavan Thangapandian a,*, Archana Jayaram b, Gunadhor Singh Okram c, Niranjan Prasad Lalla c, Milton Franklin Benial Amirtham a

*Author for correspondence: Dr. Mathavan Thangapandian., Ph. D., Assistant Professor, PG & Research Department of Physics, NMSSVN College, Nagamalai Madurai-625 019, Tamilnadu, India TEL: +91-9486953567, E-mail: [email protected]


Induced Aggregation of Steric Stabilizing Anionic ... - ACS Publications

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