Successive Surface Reactions on Hydrophilic Silica for Modified

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Successive Surface Reactions on Hydrophilic Silica for Modified Magnetic Nanoparticle Attachment Probed by Sum Frequency Generation Spectroscopy Jeeranan Nonkumwong, Uriel Joseph Erasquin, Kurt Waldo E. Sy Piecco, Uvinduni I. Premadasa, Ahmed M. Aboelenen, Andrew A. Tangonan, Jixin Chen, David C. Ingram, Laongnuan Srisombat, and Katherine Leslee Asetre Cimatu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01333 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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Successive Surface Reactions on Hydrophilic Silica for Modified Magnetic Nanoparticle Attachment Probed by Sum-Frequency Generation Spectroscopy Jeeranan Nonkumwong*†‡, Uriel Joseph Erasquin*†, Kurt Waldo Sy Piecco†, Uvinduni I. Premadasa†, Ahmed M. Aboelenen†, Andrew Tangonan†, Jixin Chen†, David Ingramⱡ, Laongnuan Srisombat*‡, and Katherine Leslee Asetre Cimatu*† †Department of Chemistry and Biochemistry, Ohio University, 100 University Terrace, 136 Clippinger Laboratories, Athens, Ohio 45701-2979, United States ⱡDepartment of Physics and Astronomy, Ohio University, 139 University Terrace, 136 Clippinger Laboratories, Athens, Ohio 45701-2979, United States ‡Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand KEYWORDS: thiol-ene reaction, surface, sum-frequency generation spectroscopy, oxide nanoparticles, solid-liquid interface

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ABSTRACT

Successive surface reactions on hydrophilic silica substrates were designed and performed to immobilize ethanolamine-modified magnetic ferrite-based nanoparticle (NP) for surface characterization. The various surfaces were monitored using sum-frequency generation (SFG) spectroscopy. The surface of the hydrophilic quartz substrate was first converted to a vinylterminated surface by utilizing a silanization reaction, and then, the surface functional groups were converted to carboxylic-terminated groups via a thiol-ene reaction. The appearance and disappearance of the vinyl (=CH2) peak at ~2990 cm-1 in the SFG spectra were examined to confirm the success of the silanization and thiol-ene reactions, respectively. Acyl chloride (COCl) formation from carboxy (-COOH) functional group was then performed for further attachment of magnetic amine-functionalized magnesium ferrite nanoparticles (NPs) via amide bond formation. The scattered NPs attached on the modified silica substrate was then used to study the changes in the spectral profile of the ethanolamine modifier of the NPs for in situ lead (II) (Pb2+) adsorption at the solid-liquid interface using SFG spectroscopy. However, due to the limited number of NPs attached and sensitivity of SFG spectroscopy towards expected change in the modifier spectroscopically, no significant change was observed in the SFG spectrum of the modified silica with magnetic NPs during exposure to Pb2+ solution. Nevertheless, SFG spectroscopy as a surface technique successfully monitored the modifications from a clean fused substrate to -COCl formation that was used to immobilize the decorated magnetic nanoparticles. The method developed in this study can provide a reference for many surface or interfacial studies important for selective attachment of adsorbed organic or inorganic materials or particles.


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INTRODUCTION Surface modification of silica-based substrates allows the alteration of the physicochemical properties that are most needed in several applications, especially in optoelectronics.1,

2, 3, 4

Reacting trialkoxy- or trichlorosilane with specific terminal groups with hydroxylated silica surfaces is one of the most common and convenient ways to tailor surface properties.5,

6

Specifically, the radical-based thiol-ene reaction, a member of the click reaction class7, is attractive among other commonly used reactions. The reaction can take place under mild conditions even in the presence of oxygen8, 9, can be run neat or in solvents6, provides high yields7, does not require catalysts10, and can use a number of commercial precursors to provide various terminal groups as needed.6, 10, 11 Successive surface thiol-ene reactions involve the stepby-step functionalization of the hydroxylated substrate. The steps involved can either be performed with alkenyl silanes, followed by reaction with thiol groups of mercaptans, or vice versa, i.e., functionalization by mercaptosilanes, followed by alkenes.12 Consequently, the modified substrates could be further applied as linker structures for a variety of organic and inorganic substances, e.g., biological molecules, macromolecules, and particles.6, 13 We are interested in employing radical-based thiol-ene reactions to prepare a modified silica substrate, namely, ethanolamine-functionalized magnesium ferrite (MgFe2O4) magnetic nanoparticles, for in-house modified and colloidal oxide nanoparticle (NP) attachment. We adopted these nanoparticles for lead (II) ion (Pb2+) removal from simulated wastewater in the previous work, by Srisombat and co-workers14 since these magnetic NPs are easily separated after Pb2+ adsorption due to their inherent magnetic properties. Under certain conditions, the 3 ACS Paragon Plus Environment

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synthesized NPs were able to efficiently adsorb ~99% Pb2+, characterized by atomic absorption spectroscopy (AAS).14 AAS is a well-known conventional technique for monitoring adsorption processes in bulk solution and is usually performed by calculating the concentration difference in the adsorbates before and after interacting with the adsorbents. 14, 15, 16, 17, 18 Indeed, such studies provide essential information for applications, i.e., how many adsorbents should be employed under certain conditions. However, apart from bulk solution studies, few studies have been performed to explain the mechanism underlying the interfacial interaction between fabricated oxide magnetic nanoparticles as adsorbents and Pb2+ ions or any other compounds as adsorbates. To further understand the role of the nanoparticle modifier in the adsorption process, it is necessary to prepare the desired functionalized flat surface to immobilize the nanoparticle for further experiments using SFG spectroscopy. Also, it is necessary because it is difficult to characterize samples in solutions especially for the case of these magnetic nanoparticles that settle to the bottom of the container after some time. Therefore, this study focuses on developing a systematic approach to immobilize magnetic nanoparticles on a dielectric substrate through multiple successive surface reactions to carry out the experiment. Second-order nonlinear spectroscopic techniques can be employed to characterize these modified surfaces and interfaces. For instance, many studies have utilized modified silica substrates for anion (CrO42-) or cation (Ca2+, Cd2+, and Zn2+) interactions at the buried solid-water interface using nonlinear optical second harmonic generation (SHG) spectroscopy. 19, 20, 21, 22, 23 The SHG signals vary depending on the interaction of the adsorbed ion species with the modified surfaces under certain conditions.19,

20, 21, 22, 23

The silica substrates were initially functionalized with

synthesized ester-terminated silanes and then were hydrolyzed to obtain carboxy-terminated monolayers.19, 20, 21, 22, 23, 24, 25 Additionally, SFG spectroscopy, a surface-sensitive technique with 4 ACS Paragon Plus Environment

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molecular sensitivity, has been utilized to monitor the conversion process.21, 24, 25 Apart from the hydrolysis of ester, other SFG studies have been performed to characterize surface click reactions.5,

26

Chen et al. functionalized glass substrates using commercially available

aminophenyl, which was then converted to azidophenyl groups by the oxidation reaction. To test the efficiency of copper-catalyzed azide-acetylene cycloaddition on the prepared azidophenylterminated substrates, they employed small terminal alkyne molecules, including acetylenemodified single-strand DNA, and analyzed the surfaces before and after reaction using X-ray photoelectron spectroscopy, fluorescence spectroscopy, and SFG spectroscopy.5 The results of these studies suggest that successive surface reactions, e.g., thiol-ene reactions, can be probed using SFG spectroscopy to initiate further attachment of surface-functionalized oxide NPs. Moreover, to the best of our knowledge, this work presents the first examination of a surface thiol-ene reaction by probing the surface before and after the reaction of each step using SFG spectroscopy. Because our synthesized MgFe2O4 NPs have an amine (R-NH2) terminal group, to attach these NPs to the dielectric silica substrate, an intermediate acyl chloride group (R’-COCl) is needed to form the amide linkage (R’CONHR) under acidic medium of pH = 4,27, 28, 29

as shown in Scheme 1. Thus, in the present work, simple and commercially available

molecules, such as vinyltrimethoxysilane (VTMS) and thioglycolic acid (TGA), were chosen as the alkenyl silane and mercaptan sources, respectively. These compounds were selected for surface thiol-ene reactions to prepare COOH-terminated silica substrates, which were then converted to COCl-terminated substrates, and to attach the magnetic NPs, respectively. In addition to probing successive surface reactions, SFG can assist in the preliminary study of investigating the interfacial changes of the NP modifiers response to adsorption of Pb 2+ ions on 5 ACS Paragon Plus Environment

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immobilized modified magnetic NPs at a solid-liquid interface. Numerous SFG studies have investigated the surface characteristics of the oxide NP layer on transparent substrates via interaction with molecular probes such as methanol30 and an ionic liquid.31 In addition to oxide NPs, one SFG study was able to differentiate between interactions of an ionic liquid with graphene and bare BaF2 surfaces. The anion preferred BaF2, while both the anion and cation parts were detected on the graphene surface.32 Therefore, with the proper fabrication of a NPattached silica substrate, we can use SFG spectroscopy to probe the solid-liquid interface between modified magnetic NPs and Pb2+ ions in an acidic medium under conditions that were also used for the bulk solution adsorption study. The assessment of the information regarding interfacial interactions between the adsorbents and adsorbates could support the proposed adsorption mechanism in the previous work.14

Scheme 1. Schematic illustration of the preparation of a MgFe2O4 NP-attached silica substrate facilitated by a surface thiol-ene reaction for the characterization of surfaces and interfaces. Sum-Frequency Background. publications.33,

34, 35, 36

The details on SFG background is available in earlier

Briefly, SFG spectroscopy is a second-order nonlinear spectroscopic 6 ACS Paragon Plus Environment

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method that provides the vibrational spectra of molecules present solely at surfaces and interfaces. This technique involves tunable infrared (IR) and fixed-frequency visible beams; these beams overlap on the surface to generate a third beam, which has the sum of the frequencies of the two incident beams. The selection rules for SFG activity are different from those required for infrared and Raman spectroscopy. The SFG activity of molecular vibrational modes must occur in a noncentrosymmetric environment and must also be satisfied at both the macroscopic and molecular levels. Thus, in the bulk phase, the isotropic distribution of molecules is centrosymmetric, which results in SFG inactivity. Then, if a region exists between two different isotropic bulk phases introduced as an interface, a plane of asymmetry is created, giving way to SF activity in the molecules present at the interface. The SFG signal is also affected by the arrangement of molecules at the surface and interface, as no emission is observed from a completely disordered surface structure. Thus, SFG spectroscopy, as mentioned earlier in the text, has interface specificity, which makes the signal distinguishable from the bulk medium. At a molecular level, the SFG signal is also affected by the conformation of the interfacial molecules. SFG is a coherent process, and the SFG light can be described as having a magnitude, direction, and phase. Overall, SFG spectroscopy allows the determination of the average tilt angle and distribution of tilt angles of the interfacial molecules. Moreover, the intensity of the sum-frequency beam is proportional to the square of the induced second-order nonlinear polarization. The induced polarization response is related to the electric fields through the second-order nonlinear susceptibility tensor, 𝜒 (2) . In addition, the secondorder nonlinear susceptibility has resonant and nonresonant contributions. The nonresonant influence (𝜒𝑁𝑅 (2) ) is intrinsic to the substrate, whereas the resonant contribution (𝜒𝑅 (2) ) contains 7 ACS Paragon Plus Environment

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the vibrational modes of interfacial molecules. Dielectric materials, such as silica37, are considered to have an almost negligible nonresonant susceptibility. (2)

Therefore, the intensity of the SFG signal is mainly dependent on the resonant term 𝜒𝑅 which is related to hyperpolarizability 𝛽, the product of the IR dipole moment and Raman polarizability tensor. 𝐼𝑆𝐹𝐺 ∝ |𝜒

(2) 2

| ∝ |𝜔

𝑁 𝑞 −𝜔𝐼𝑅 +𝑖𝛤𝑞

+

2 (2) 𝑖𝜌 |𝜒𝑁𝑅 |𝑒 |

(1)

N is the number density of vibrational transitions and 𝛤𝑞 is the damping constant of the qth vibrational mode. 𝜔𝑞 and 𝜔𝐼𝑅 are the resonance and the incident IR frequencies, respectively. 𝜌 is the phase of the non-resonant response. The non-resonant contribution is considered in the fitting equation, in order to account for any contribution from the bulk. The simplified version of the fitting equation is shown below. 𝐼𝑆𝐹𝐺 (𝜔 + 𝜔𝑣𝑖𝑠 ) ∝ 𝑒𝑥𝑝 [−

𝐿 2 (𝜔−𝜔𝐼𝑅 )

2(𝛿𝜔𝐿 )2

] × |∑𝑞 𝜔

𝐴𝑞 𝐼𝑅 − 𝜔𝑞 +𝑖Γ𝑞

2

+ 𝐴𝑁𝑅 𝑒 𝑖𝜌 |

(2)

The contribution from the broadband width of the IR beam profile is considered by including the 𝐿 34, 38 Gaussian function with the spectral width of 𝛿𝜔𝐿 centered at 𝜔𝐼𝑅 . The amplitude factors, 𝐴𝑞

and 𝐴𝑁𝑅 , are proportional to 𝛽 as shown in Equation 1. EXPERIMENTAL METHODS Materials. Magnesium nitrate hexahydrate (Mg(NO3)26H2O) (99%) and sodium acetate (CH3COONa) (99.5%) were purchased from Loba Chemie (Mumbai, India). Iron(III) nitrate nonahydrate (Fe(NO3)39H2O) (98%), ethanolamine (99%), ethanol (95%), nitric acid (HNO3) (68-70%), NaOH (97%), 200-proof ethanol (100%), thionyl chloride (SOCl2) (99%), and D2O 8 ACS Paragon Plus Environment

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(99.9%) were supplied by Carlo Erba (Val-de-Reuil, France), BDH Limited Poole England (Radnor, PA), J.T. Baker (Georgetown KY), Acros Organics (Pittsburgh PA), VWR International (Radnor, PA), Decon Labs (Prussia, PA), Alfa Aesar (Haverhill, MA), and Cambridge Isotope Laboratories Inc. (Tewksbury, MA), respectively. Ethylene glycol (99.5%), sulfuric acid (H2SO4) (97%), toluene (99.5%), 2-propanol (99.5%), acetone (99.5%), and dichloromethane (DCM) (99.9%) were purchased from Fisher Scientific (Pittsburgh PA). VTMS (98%), TGA (98%), 2,2-dimethoxy-2-phenylacetophenone (DMPA) (99%), lead (II) nitrate (Pb(NO3)2) (99%), and deuterium chloride (DCl) solution (35 wt.% in D2O, 99 atom% D) were purchased from Sigma-Aldrich (St. Louis, MO). All chemicals were used as received without further purification. Silica (fused quartz) windows (round, 2 in. diameter, 1/8 in. thickness) were purchased from Esco Optics, Inc. (Oak Ridge, NJ). Deionized (DI) H2O (18.1 MΩ·cm) was used throughout the experiments. A. Synthesis of Ethanolamine Surface-Modified MgFe2O4 NPs. MgFe2O4 NPs with a particle size of ~88±9 nm were synthesized, and ethanolamine was employed simultaneously as a surface modifier to functionalize the MgFe2O4 NPs using the preparation procedures previously reported.14 Briefly, 15 mmol CH3COONa dissolved in 20 mL ethylene glycol was heated to 100 °C, stirred and refluxed for 15 minutes. A dissolved solution of 1 mmol Mg(NO3)26H2O and 2 mmol Fe(NO3)39H2O in 10 mL ethylene glycol was poured into the preheated solution of CH3COONa and then stirred for 30 minutes before adding 60 mmol ethanolamine. The mixture was then heated to 200 °C, maintained for 12 hours, and then naturally cooled to room temperature (RT). The obtained precipitates were collected by a magnetic separator, washed with DI H2O at least ten times, and then washed with 200-proof ethanol twice. The final product was 9 ACS Paragon Plus Environment

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dried at 70 °C for 12 hours. The details of NP characterization were provided in the previous publication.14 B. Preparation of Ethanolamine Surface-Modified MgFe2O4 NP-Attached Silica Substrate. Silica Substrate Preparation. Silica windows were cleaned using the procedures previously reported.39 The windows were soaked in a H2SO4/HNO3 50/50 vol. mixture overnight. The silica windows were rinsed with DI H2O, dried with nitrogen (N2) gas, and dried in an oven for ca. 30 minutes. Afterward, the silica windows were transferred to 2 M NaOH for 15 minutes at RT, rinsed with excess DI H2O, dried neatly with N2 gas, and plasma cleaned. All the substrates were used immediately after cleaning. Surface Modification of Silica Substrates with Self-Assembled Monolayers (SAMs) of VTMS. Monolayer films of VTMS were created on the cleaned silica surface by adapting procedures from previously reported studies. 40 The clean silica substrate was soaked in an 8% (w/w) anhydrous toluene solution of VTMS in a covered glass petri dish and sealed with Parafilm. After ~40 hours at ambient atmosphere, the solution was decanted. The substrate was rinsed with fresh toluene at least 2 times and baked in an oven at 110 °C for 1 hour.24 Conversion of Vinyl-Terminated to COOH-Terminated Silica Substrate (Thiol-Ene Reaction). The conversion procedures were adopted and slightly modified from previous publications.40, 41, 42, 43, 44 In brief, 400 µL of 2% (w/w) DMPA in neat TGA was placed in the glass petri dish, and then the precharacterized vinyl-terminated substrate was immediately placed in the solution. The substrate in the petri dish was irradiated with 110 V 20 W UV LED light (power density of 4.8 x 10

-5

W/cm2, Sunlite SL20 BLB lamp) (λmax ~395-400 nm). The power

density was calculated by considering the size of the silica substrate. After 15 minutes at ambient atmosphere, the substrate was rinsed with 2 cycles of DI H2O and 200-proof anhydrous ethanol 10 ACS Paragon Plus Environment

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to remove excess TGA, followed by 2-propanol and acetone to remove excess DMPA; the substrate was then baked in an oven at 110 °C for 1 hour. Then, the sample was cooled down prior to acyl chloride formation in the next step. Conversion of COOH-Terminated to COCl-Terminated Silica Substrate (Acyl Chloride Formation) The sealed reaction vessel with COOH-terminated silica substrate (precharacterized using SFG) in 350 mL DCM was purged with N2 gas for 2 minutes before the dropwise addition of excess SOCl2 (1.3 mL, 17.9 mmol) in 10 mL DCM under magnetic stirring. The reaction was left at RT for 16 hours, and 0.1 M NaOH was used to trap the acid gas during the reaction (the setup is shown in the Supporting Information, Figure S1).27, 45, 46, 47 After 16 hours of reaction, the remaining SOCl2 and DCM were decanted, and the substrates were rinsed with fresh DCM twice. The substrates were used immediately for the next step, since acyl chloride is moisture sensitive, and thus, the surface can react partially with water and form COOH-terminated silica substrate reversibly under the right conditions.48 One of the as-prepared COCl-terminated substrates was also characterized using SFG spectroscopy at the air-solid interface under ambient conditions before immobilization of the NPs. Immobilization of Ethanolamine Surface-Modified MgFe2O4 NPs onto the COClTerminated Silica Substrate. The coupling reaction of R-NH2 with R’-COCl, adapted from Lockett et al., was performed.27 Immediately after obtaining the COCl-terminated silica substrate, 350 mL DCM with 25 mg of NPs that had been sonicated for 5 minutes was transferred to the nitrogen-purged reaction vessel containing COCl-terminated silica substrate under magnetic stirring. After 1 hour at RT, the solvent was decanted, and the substrate was baked in an oven at 110 °C for 1 hour to remove residual DCM solvent completely. (Note: Based


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on one reference49, heating the surface-modified nanoparticles to less than 170 °C will not remove the ethanolamine). C. Instrumentation. The detailed setup for SFG spectroscopy (Figure S2) was described in an earlier publication by the group.50 The SFG experiments were performed in an ambient environment with a relative humidity of ~30±2% and room temperature of ~21.2±1.5 °C. In this work, the spectra were collected with ssp (an indexing order of s-polarized SFG output, s-polarized visible input, and ppolarized IR input) and ppp polarization combinations, where s denotes that the polarized light is perpendicular to the plane of incidence, and p denotes that the polarized light is parallel to the plane of incidence. The ssp polarization combination probes modes in which the components of the IR transition moment are perpendicular to the interfacial plane. Moreover, the ppp polarization combination probes all components of the allowed vibrations; that is, ppp-polarized SFG spectra exhibit vibrational modes with both perpendicular and parallel components.51, 52, 53 For the solid-air interface study (Figure S3), freshly prepared substrates were preferred, to avoid any degradation or contamination upon exposure to the ambient conditions of the external environment. Each spectrum was acquired for 3 s of exposure time for 180 acquisitions (total 9 minutes/one replicate) at each IR center (1600, 1750, and 2800-3700 cm-1). The centers at 1600 cm-1 and 1750 cm-1 probe the C=C and C=O vibrational regions, respectively. Additionally, the 2800-3700 IR centers probe the CH, OH, and the NH vibrational regions. All spectra were collected for three trials with one background. The input energies to the sample surface were ~47 μJ/pulse from the visible beam and ~24 μJ/pulse from the IR beam centered at 2900 cm-1.


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For the solid-liquid interface study, an in-house chemical-resistant SFG sample cell was built and designed to keep the liquid inside while the experiment was running. Two silica windows with O-rings were needed to seal the cell, but the cell still retained optical accessibility.36 Information was collected at the interface between the modified substrate (i.e., NP-attachedsilica) and the solution (i.e., D2O and Pb2+ in D2O at pD ~4.4) inside the sample cell, as shown in Figure S4. Approximately 25 mL of solution was injected via syringe into the sample cell through the inlet port. Spectra were acquired for 3 s of exposure time for 300 acquisitions (total 15 minutes/one replicate) at one IR center (2900 cm-1). All spectra were collected three times, with the addition of another acquisition for background correction. The input energies to the solid-liquid interface were ~37 μJ/pulse and ~24 μJ/pulse from the visible beam and IR beam, respectively. The input energy for the fixed visible beam was reduced to avoid localized heating. Other Techniques. 1H nuclear magnetic resonance (NMR) spectroscopy (Bruker 500 MHz spectrometer, Billerica, MA) was used to assess the purity of the starting materials. Fourier transform infrared (FTIR) spectrometry (Shimadzu IRAffinity-1S, Columbia, MD) was employed to characterize the neat VTMS liquid to assist with the peak assignment of the vinylmodified silica substrate. To study the distribution of the NPs on a silica substrate, the NPattached substrate was analyzed by atomic force microscopy (AFM) (Asylum Research MFP3D-SA, Santa Barbara, CA) using tapping mode. A silicon 7 nm cantilever tip with a spring constant of k = 26 N·m−1 and the resonant frequency of ~272 kHz was used.54 To reduce agglomeration and aggregation, the preparation of the NP-attached substrate for AFM analysis was slightly different from that used for SFG spectroscopy. The NP powders in DCM after sonication were filtered using a PTFE filter syringe with pore size ~220 nm (Simsii, Inc., Port 13 ACS Paragon Plus Environment

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Irvine, CA) before drop-casting on the COCl-terminated substrate. A lower-coverage-density sample was prepared to obtain an image of well-separated magnetic NPs on the surface. AFM images were collected with a scan area of 20 x 20 µm2. X-ray Photo-electron Spectroscopy (XPS) was performed on the samples with a Kratos XSAM800 with an aluminum x-ray source part of the Keck Thin Film Analysis Facility at Ohio University. Pass energy of 40 eV was used for the 125 mm radius spectrometer. All of the samples charged 4V to 5V during analysis. Rutherford Backscattering Spectroscopy (RBS) was used to measure the amount of lead on the surface of the sample. RBS utilized a beam of 2.2 MeV helium ions from the 4.5 MV tandem accelerator of the Edwards Accelerator Laboratory of Ohio University. The beam was at normal incidence to the target and scattered through 168 degrees into a solid state detector with an energy resolution of about 15 keV.

RESULTS AND DISCUSSION First, the vinyl-terminated silica was prepared, and then, the successive conversion of the vinyl groups to -COOH and -COCl groups was performed; finally, NP attachment was performed. Each substrate was characterized by SFG spectroscopy at the solid-air interface to monitor the surface molecular structural changes. The NP-decorated silica was used to obtain preliminary results for the in situ Pb2+ adsorption study at the solid-liquid interface. Successive Surface Reaction Monitoring: Vinyl-Terminated Silica. The appearance of vibrational peaks for the vinyl group is essential to verify the first step of the surface 14 ACS Paragon Plus Environment

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functionalization of the silica substrate. The main peaks (shown in Figure 1A (blue highlight) and tabulated in Table 1) of vinyl-terminated silica in the CH stretching regions are positioned at ~2990 cm-1 (CH symmetric stretching (SS) of =CH2)55, 56, 57, 58, 59, 60, 61, 62 when using the ssp polarization combination. Additionally, the peaks positioned at ~3020 and ~ 3060 cm-1 were assigned to =CH stretching55, 56, 60, 62, 63 and =CH2 asymmetric stretching (AS)55, 56, 57, 58, 59, 60, 61, 62, 64, 65

, respectively, using the ppp polarization combination.

These peaks confirmed the surface-bound vinyl groups and the successful surface modification of clean hydrophilic silica terminated with silanol (-Si-OH) groups. Additionally, in the SFG spectra collected at an IR center of 1600 cm-1, shown in Figure 1B, the intense peak at approximately 1605 cm-1 (ssp) is from the C=C stretching of the vinyl group, which further proves the existence of the vinyl-terminated silica surface.60, 61, 62, 64, 65, 66, 67, 68, 69, 70, 71 In Figure 1A, the peak at ~2899 cm-1 (ssp) was assigned to the unreacted –OCH3 SS of symmetrical SiOCH3, which indicates the linear or nearly linear conformation of SiOC72. The peak at ~2852 cm-1 (ssp) could be assigned to the unreacted methoxy (–OCH3) SS of asymmetrical SiOCH3,61, 66, 67, 72

which indicates the bent conformation of SiOC72.

PPP SSP Fitdata

0.4

SFG Intensity (a.u.)

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

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0.3

0.2

0.1

0.0 2600

2800

3000

3200

3400

3600

3800

Wavenumber (cm-1)

(A) 15 ACS Paragon Plus Environment

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0.12

PPP SSP


0.10 0.08 0.06 0.04 0.02 0.00 1400

1500

1600

1700

1800

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(B) 0.12

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0.10

SFG Intesnity (a.u)

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

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0.08 0.06 0.04 0.02 0.00 1600

1700

1800

1900 -1

Wavenumber (cm )

(C) Figure 1. SFG spectra of the air/vinyl-terminated silica interface using with the IR beams centered at (A) 2800-3700 cm-1, (B) 1600 cm-1, and (C) 1750 cm-1 at ssp and ppp polarization combinations. See narrative for the blue-colored region in (A). In addition, the ~2844 cm-1 peak can be influenced by the presence of an impurity (R-Si-CH2CH3, e.g., ethyltrimethoxysilane73 from manufacturing) in neat VTMS, as shown in the 1H NMR spectra (Figure S5 and S6) (1H NMR (CDCl3): δ = 0.61 (CH2, q, 2H, J = 8.0 Hz), 0.97 (CH3, t, 3H, J = 8.1 Hz)). The ~2957 cm-1 peak (ppp) was assigned to the unreacted –OCH3 AS of the asymmetrical SiOCH3.61, 66, 67, 72 Additionally, we have provided an extended SFG spectrum of vinyl terminated functionalized surface from 1400 cm-1 to 2000 cm-1 in the supporting information (Figure S7 A). The broad SFG peak observed at ~3200 cm-1, and ~1800-1900 cm-1 16 ACS Paragon Plus Environment

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Langmuir

could be originating from the silica substrate. To support this observation, we also collected the spectra from a clean silica substrate (Figure S8). Due to the similarity in the spectral profile, it was evident the broad peak at ~ 1800 cm-1 has a contribution from the silica substrate. Moreover, the resonant spectra of the vinyl-terminated silica were obtained at IR centers of 3200-3300 cm-1 and 1600-1700 cm-1 (Figure S9) to reduce the background. The nonresonant spectrum was obtained at t = 0, while the resonant spectra were obtained by adjusting the timing delay of the 795 nm beam asymmetric pulse (0.6, 1.2, 1.8, and 2.4 picoseconds) using a Fabry-Perot etalon.74 The only observable peaks in the resonant SFG spectra are the vibrational modes from the vinyl group (CH SS and CH AS of =CH2 and C=C stretching), as shown in Figure S9. The resonant spectra acquired at different delays showed a subsequent background reduction. The overall SFG counts were also affected, but the signal-to-noise ratio was still acceptable. Thus, the spectral profile became more evident. The peak observed at ~ 1450 cm-1 (Figure S7 A) is also originating from the silica substrate as observed in the spectrum of clean silica (Figure S8). The acquired IR spectrum of the neat VTMS shown in Figure S10 and the certificate of origin (FTIR) provided by the vendor are consistent with the SFG results.75 The presence of CH vibrational modes other than C-H stretching from the vinyl group indicates a less than 100% conversion of the surface silanol groups to vinyl groups. This step is critical to the success of the subsequent steps. In general, the SFG signal is affected by the organization of the molecules, and the number of these probed functional groups at the surface. Therefore, probing the full conversion of the silica surface using SFG spectroscopy is dependent on many factors. In this specific step, 1) the pristine condition of the unmodified silica surface and 2) humidity and temperature, in effect, influence the surface


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modification of these silanol groups to vinyl groups. Thus, the presence of other peaks besides the –C=CH2 vibrational modes is apparent in Figure 1. COOH-Terminated Silica. SFG data showed that the hydrophilic silica was successfully terminated with vinyl groups. Therefore, proceeding with the thiol-ene surface reaction (Figure 1), the vinyl peaks should disappear upon exposure to TGA under SFG measurement. As expected, the =CH2 SS, =CH stretching, =CH2 AS and C=C peaks (yellow highlight) were significantly reduced, with an almost negligible contribution to the SFG spectrum obtained from the COOH-terminated substrate, as the conversion of vinyl- into COOH-terminated groups approached completion (Figure 2A). The peak centered at ~2855 cm-1 (ssp and ppp) in Figure 2A could be assigned to unreacted –OCH3 SS61, 66, 67, 72 from VTMS and CH2 SS from the CH2 group between the S atom and C=O group (HOOC-CH2-S-R).76 Unlike the vinyl-terminated sample, there is another distinct peak centered at ~2873 cm-1 (ssp), which was assigned to the CH2 SS vibrational mode of the CH2 groups between S and Si atoms (R’-S-CH2-CH2-Si-R’’)77. The peak centered at ~2904 cm-1 (with a zoomed-in spectrum in Figure S11) in the ssp spectrum was assigned to the CH2 Fermi-related (FR) mode of the CH2 group positioned between the S atom and C=O group.76 The peak positioned at ~2932 cm-1 (ssp) can also be assigned to CH2 FR but is from the CH2 group located between the S and Si atoms. The broad-peak SFG signal positioned at ~3232 cm-1 was assigned to the OH stretching from the -OH group of -COOH. In the ppp spectrum, the 2883 cm-1 peak can be assigned to CH2 AS from the CH2 group adjacent to the carboxylic group, as presented in the literature by Asanuma et al.53 The peak positioned at ~2962 cm-1 (ppp) can have contributions from the –OCH3 AS of unreacted –OCH3 groups61, 66, 67, 72

and

CH2

AS

from

the

CH2

group

between

the

S

and

Si

atoms78,

79

.


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Table 1. Summary of the vibrational modes of the vinyl, COOH, COCl, and NP-attached surfaces and their peak assignments*. Vinyl COOH COCl NP-attached Reference Number ssp ωi ppp ωi ssp ωi ppp ωi ssp ωi ppp ωi ssp ωi ppp ωi SFG IR Raman -1 -1 -1 -1 -1 -1 -1 -1 Vibrational assignment (cm ) (cm ) (cm ) (cm ) (cm ) (cm ) (cm ) (cm ) Unreacted–OCH3 SS61, 66, 67, 72 ~2852 57,62, 63,69 CH2 SS between the S atom ~2855 ~2849 ~2853 ~2846 72 76 and C=O group CH2 SS adjacent to the O ~2855 ~2854 73-75 atom of functionalized ethanolamine78, 80, 81 CH2 SS between S and Si ~2873 ~2865 77-78 76 atoms77, 82, 83 CH2 SS adjacent to the N ~2878 74, 80 79 atom of functionalized ethanolamine79, 80, 84 CH2 AS between the S atom ~2883 ~2881 73 78 and C=O group CH2 AS adjacent to the O ~2890 73 atom of functionalized ethanolamine78 Unreacted –OCH3 SSa72 ~2899 69 CH2 FR between the S atom ~2904 ~2912 ~2905 ~2916 72 76 and C=O group CH2 FR adjacent to the O ~2915 ~2925 73 atom of functionalized ethanolamine78 CH2 FR between S and Si ~2932 ~2929 81 79 atoms79, 85 CH2 FR adjacent to the N ~2934 81 79 atom of functionalized ethanolamine79, 85 19 ACS Paragon Plus Environment

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Unreacted–OCH3 AS61, 66, 67, 72 CH2 AS between S and Si atoms78, 79 CH2 AS adjacent to the N atom of functionalized ethanolamine78, 79 =CH2 SS55, 56, 57, 58, 59, 60, 61, 62

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~2951

57,62, 63,69 ~2962

~2964 ~2960

~2990

=CH55, 56, 60, 62, 63 =CH2 AS55, 56, 57, 58, 59, 60, 61, 62,

73

79

73

79

~2980

51,55

~3020 ~3060

51,59 51

~1595

64-66

64, 65

C=C stretching60, 61, 62, 64, 65, 66,

~1605

67, 68, 69, 70, 71

NH2 bending5, 79, 80, 86 C=O stretching, cyclic dimer76 C=O stretching, acyclic ~1731 dimer71, 76, 87, 88 C=O stretching, monomer76 *Fitting parameters for are available in Table S1 in the SI. a –OCH3 of symmetrical SiOCH3.72

~1610 ~1700

~1713 ~1747

~1777

~1615

~1748 ~1785

~1778

5,74

52-54, 57,58 52, 58 52-54, 57,58, 61 57, 58,6063,67 82

52,56 52,56 52,56, 60 56,60, 67 79,82 72 67,72, 83,84 72


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PPP SSP Fitdata


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(B) 0.10

PPP SSP

0.08


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

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0.06 0.04 0.02 0.00 1600

1700

1800

1900

Wavenumber (cm-1)

(C) Figure 2. SFG spectra of the air/COOH-terminated silica interface with the IR beams centered at (A) 2800-3700 cm-1, (B) 1600 cm-1, and (C) 1750 cm-1 at ssp and ppp polarization combinations

21

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The blue region denotes the peak position of the vinyl group in the first step. The intensity becomes a spectral marker of conversion. After reacting the vinyl-terminated silica substrate with TGA, the C=C stretching peak (~1605 cm-1) was reduced, as shown in Figure 2B. The C=O peak is observed at ~1727 cm-1 in the ssp spectrum and two convoluted peaks positioned at ~1700 cm-1 and ~1777 cm-1 are obtained using the ppp polarization combination. In the extended spectra of -COOH functionalized substrate (See Figure S7B), the -C=O is still present, as shown in the inset, but convoluted with some residual peaks. The peak positioned at 1605 cm-1 in both the ssp and ppp spectra could be due to the unreacted vinyl groups and/or silica substrate signal. To verify the source of these peaks, ssp and ppp spectra of a clean silica substrate were acquired (see Figure S8). As shown, a similar spectral profile was obtained for clean silica. However, the SFG counts are lower. The only difference between the clean silica and the -COOH modified surface is the presence of the shoulder in Figure S7B. These residual peaks/nonresonant background from the silica determine the peak position and interpretation much more difficult in both ssp and ppp spectra. Due to lower SFG counts, the background affected the overall spectral profile of a -COOH modified substrate. The positions of the observed peaks are significantly different from the peak positions for the vinyl-terminated surface characterized in the same spectrum range. According to the literature, the peaks centered at ~1700 cm-1 (ppp) and ~1731 cm-1 (ssp) can be assigned to C=O stretching from cyclic76 and acyclic71,

76, 87, 88, 89, 90

dimers, respectively, which is a result of

hydrogen bonding between the terminal carboxylic acid groups. The peak positioned at ~1777 cm-1 can be assigned to non-hydrogen-bonded C=O stretching.76 Again, the broad SFG signal affecting the baseline could be due to the nonresonant background signal innate to the silica 22


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Langmuir

substrate and/or adsorption of water90. The peaks obtained from the ssp polarization combination at both ~1600 and ~1750 cm-1 show that the thiol-ene reaction and the conversion have taken place. Again, the full conversion to -COOH is affected by the coverage of –C=CH2 in the previous step. COCl-Terminated Silica. To attach the ethanolamine-functionalized NPs, the substrate surface needed to be modified again from COOH- to COCl-terminated groups. This modification allows the formation of an amide bond (R’CONHR) to anchor the NPs onto the modified silica substrate. Predictably, the peak profile and peak position in the CH stretching region of the COCl-terminated silica substrate are similar to those of the COOH-terminated silica substrate, since the scale and position of only the terminal group were changed (Figure 3). Noticeably, the COCl-terminated silica also provides a significant OH signal (~3200 cm-1), as shown in Figure 3A, indicating that the -COCl group is moisture sensitive, which could cause the -COCl modified silica substrate to revert to the COOH-terminated form.48 The SFG spectra obtained from the surfaces terminated with -COOH and -COCl are similar; in particular, –C-Cl stretching (~771 cm-1) is not detectable and not within the IR range of 4000-1000 cm-1.91, 92, 93, 94 Therefore, to increase the NP attachment as much as possible and prevent the reversion of -COCl to -COOH, amide bond formation was performed immediately after obtaining the -COCl terminated silica substrate, as mentioned earlier in the experimental section. In Figure 3B, the peak observed at ~1748 cm-1 (ssp) can be assigned to the C=O stretching of the -COCl group. After conversion from COOH- to COCl-terminated silica substrate, the C=O peaks were blueshifted by ~17 cm-1, showing the changes in the environment of the C=O bonds, which was consistent with the observations by Ji et al.95 The ssp polarization combination obtained a peak positioned at ~1748 23


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cm-1 while two peaks were observed from the ppp polarization combination, as described in the previous section. The extended spectra of -COCl in the region 1400-2000 cm-1 is provided in Figure S7C. The peak positioned at ~ 1600 cm-1 in both ssp and ppp spectra could originate from the unconverted vinyl groups and/or signal from the clean silica. The spectral profile is similar to -COOH modified surface mainly because of the silica background. However, the SFG spectra of the -COCl modified surface has a lower signal-to-noise ratio compared to -COOH terminated surface. This can be interpreted as a result of the partial conversion during the surface reaction or the quick reversion after exposure to humidity. These two possible scenarios could affect the surface coverage and overall SFG signal. A closer look at the -COCl spectra (inset of Figure S7C) still showed the C=O peak.

24


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0.6

PPP SSP Fitdata

0.5 0.4 0.3 0.2 0.1 0.0 2600

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PPP SSP 0.06

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PPP SSP SFG Intensity (a.u.)

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

Langmuir

0.06

0.04

0.02

0.00 1600

1700

1800

1900

Wavenumber (cm-1)

(C) Figure 3. SFG spectra of the air/COCl-terminated silica interfaces with the IR beams centered at (A) 2800-3700 cm-1, (B) 1600 cm-1, and (C) 1750 cm-1 at ssp and ppp polarization combinations. The blue region denotes the peak position of the vinyl group as a spectral marker of conversion. 25


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NP-Terminated Silica Surface. After attaching the NPs to the freshly made -COCl surface, the C=O signal was reduced compared to that of the COCl-terminated substrate (Figure 4C). The signal-to-noise ratio is low because of possible optical interference due to the attachment of the NPs once amide C=O bonds were formed, as shown in Scheme 1, or a limited number of NPs attached. For this NP-attached substrate, we assumed that the substrate was not homogeneous because of the less than 100% conversion from -COOH to -COCl, which further reduces the possibility of obtaining a better surface coverage of the NPs. The AFM image (Figure 5) confirmed the attachment of the modified magnetic NPs on the modified silica substrate, and consequently, also confirmed that any SFG signal from the modified magnetic NPs contributed to the overall SFG spectrum because the incident beam diameters are larger than the scan size.50 Therefore, spectra collected at various spots were reported for the solid-air interface, as shown in Figure S12.

26


Page 27 of 44


0.4

SSP PPP Fitdata

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PPP SSP


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1500

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(B) 0.05

PPP SSP

0.04


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

Langmuir

0.03 0.02 0.01 0.00 1600

1700

1800

1900

Wavenumber (cm-1)

(C) Figure 4. SFG spectra of the air/NP-attached silica interface with the IR beams centered with the IR beams centered at (A) 2800-3700 cm-1, (B) 1600 cm-1, and (C) 1750 cm-1 at ssp and ppp

27


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Page 28 of 44

polarization combinations. The blue region denotes the peak position of the vinyl group as a spectral marker of conversion. For the SFG peak profile of this sample, the same vibrational mode assignments in the CH stretching region as those used for the COCl-terminated substrate could be applied, since most of the organo-layer structures on the silica were not removed after attaching the NPs. Thus, CH2 SS, CH2 FR, and CH2 AS from the CH2 group adjacent to the O atom of the ethanolaminefunctionalized NP can be assigned to the peaks positioned at ~2855 cm-1 (ssp and ppp)78, 80, 81, ~2890 cm-1 (ppp)78, and ~2925 cm-1 (ppp)78, respectively (Figure 4A). The peaks located at ~2878 cm-1 (ssp), ~2934 cm-1 (ssp), and ~2971 cm-1 (ppp) were assigned to the CH2 SS, CH2 FR79,

85

, and CH2 AS78,

79

of the CH2 group adjacent to the N atom of the ethanolamine-

functionalized NP, respectively (Figure 4A). Moreover, the peak positioned at ~1613 cm-1 was assigned to the NH2 bending mode from the terminal NH2 group of the ethanolamine attached to the magnetic NP (Figure 4B). Figure S13 shows a comparison of the substrate and clean silica SFG spectra to distinguish the signal of the bending mode from the background provided in the SI. Additionally, the convoluted peak positioned at ~1728 cm-1 could be attributed to the stretching of the amide C=O that covalently bonds the modified silica substrate and the ethanolamine-decorated magnetic NP (Figure 4B). In the extended spectra of the NP-attached substrate in the region 1400-2000 cm-1 (Figure S7D), spectral interpretation is deemed difficult due to the low signal-to-noise ratio. This sample was also prepared separately from the modified substrate used in Figure 4. As noted earlier, each stage of this conversion is influenced by the conversion efficiency of the subsequent steps. Thus, the residual peaks could originate from the bare clean silica substrate (Figure S8). 28


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

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Figure 5. Representative AFM image of the NP-attached silica substrate. As mentioned, the NP-attached silica substrate was prepared to investigate any relevant spectroscopic changes to the modifier or the modified substrate, in general, for in situ Pb2+ adsorption at the NP-attached silica -deuterium oxide interface. Demonstration of in situ Pb2+ adsorption at the NP-attached silica-water interface. In Srisombat and colleagues previous publication14, ethanolamine-functionalized MgFe2O4 NPs were used for Pb2+ removal from simulated wastewater in a bulk solution experiment. It was proposed that the functionalized NH2 groups on the synthesized NPs cause the effective Pb2+ adsorption via a cation displacement process at pH = 4.14 As we developed the surface for the nanoparticle attachment, testing the ability of the immobilized nanoparticle is still at the initial stages of experimentation. We applied other surface techniques to complement SFG results. The approach we employed was we initially probed the NH stretching vibrational modes (collected at an IR center of 3400 cm-1) from the ethanolamine molecules while performing Pb2+ adsorption using SFG spectroscopy at the solid-D2O interface to determine the changes in SFG spectra with and without Pb2+ ions. However, because of the weak signal from the solid-liquid interface compared to that from the solid-air or air-solid interface, the vibrational peak at 3400 cm-1 at the solid-liquid interface was not observed (Figure S14). The weak signal at the solid-liquid interface is most likely due to 1) flat window geometry used for the solid-liquid interface 29


Langmuir

experiment affecting the overall SFG signal and 2) sparse distribution of the ethanolaminemodified magnetic nanoparticles. These two reasons played a role in the difference and decrease of SFG signal from solid-air to the solid-liquid interface. Therefore, the CH stretching region (collected at an IR center of 2900 cm-1) was probed instead, since the signal from this region is much higher than that of the NH stretching region, as shown in Figure 4A. The data can also be used as a reference for finding and optimizing the SFG signal. Moreover, CH2 groups that are adjacent to NH2 groups will be affected when the NH species from ethanolamine are changed by Pb2+ (e.g., changing from 2(R-CH2-NH3+) into 2(R-CH2-NH2)Pb2+, as proposed in eq (4) of the previous publication14). A Pb2+ concentration of ~0.03 mM was chosen in this study to ensure an excess of Pb2+ to facilitate the efficient adsorption of Pb2+. The SFG spectra of the NP-attached substrate in contact with air, D2O, and Pb2+ in D2O are shown in Figure 6.

-1

-1

2875 cm 2905 cm-1

2888 cm 2959 cm-1 Solid-Air

0.03

0.06

Solid-Air

0.04

0.02

0.02

0.00

Solid-D2O

0.02

at pD = 4.4

0.01

0.00 0.020

2+

Solid-Pb at pD = 4.4

0.015 0.010


0.01


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

Page 30 of 44

0.00 0.025

Solid-D2O

0.020

at pD = 4.4

0.015 0.010 0.005 0.000 2+

Solid-Pb at pD = 4.4

0.02

0.01

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Wavenumber (cm )

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(a) (b) Figure 6. Solid-air and solid-liquid interface SFG spectra collected at an IR center of 2900 cm-1, where the solid is the NP-attached substrate, and the liquid is D2O and Pb2+ solution at pD = 4.4, using the (A) ssp and (B) ppp polarization combinations. The Pb2+ concentration is ∼ 0.03 mM. There are two possible scenarios to explain the peak positioned at 2905 cm-1 during exposure to D2O. First, the peaks observed in the ssp spectra of the solid-D2O and solid-Pb2+ interfaces can be a result of blueshifting of the 2875 cm-1 peak present at the ssp spectrum obtained from the solid-air interface. This could be explained by a change in the chemical environment of the NPs from air to liquid surfaces36 and hydrogen bond interactions between amine groups on the NPs and D2O.96 However, the spectra of both solid-liquid interface cases were very similar, despite the peak shift due to metal ion dependence that should have been observed after adding Pb2+, as reported in other SFG studies in various systems97, 98. This might be because the CH2 vibrational modes we probed are not considered charged species, as was COO- in Robertson et al 97., or do not possess lone pair electrons, as does OH, which could be directly affected by hydrogen bonding, as reported in Casillas-Ituarte et al.98 Therefore, the change might not be very apparent compared to changes that might be observed by probing those species directly. Also. another explanation for these observations is first, the 2905 cm-1 peak is more apparent at the solid-liquid interface compared to other peaks that were visible at the solid air-interface. However, the shift from 2875 cm-1 to 2905 cm-1 might not be as consistent as it is because it could be assigned as another vibrational mode if other peaks are not detected due to low SNR. One of the future work is to improve the signal-to-noise ratio by changing the geometry of the solid substrate to enhance further the SFG signal acquired for this specific chemical system.

31


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Additionally, the SFG spectra of OD vibrational region (2500 - 2700 cm-1) were also recorded to investigate any difference in the OD vibrational modes in the presence and absence of Pb 2+ (Figure S15). The broad peak at ~2584 cm-1 can be assigned as the O-D vibrational band where interfacial D2O molecules are less ordered. As shown by the spectra from the OD vibrational region, there was no change in the hydrogen bonding network when Pb 2+ is present because there is no peak shift compared to the presence of pure D2O. The reduction in the intensity of the spectrum in the presence of 0.03 mM Pb2+ can be attributed to the reduction in the number of OD vibrational modes/molecules of D2O at the solid-liquid interface. Further surface analysis of the functionalized substrates was performed using x-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectroscopy. In XPS, the graphite C1s line was used to measure the energy shift required to offset the charging of the samples. The first step required modification of the silica substrate using VTMS. The VTMS compound has –C=CH2 group. With the careful preparation of the vinyl surface-modified silica, the % conversion to C=C (C-H) was estimated to be ~79.2%. The remaining 20.8% could come from (1) unreacted Si-OH of the silica, (2) non-conversion of 1 or 2 of the methoxysilanes from VTMS, and (3) adventitious carbon contamination. However, unreacted Si-O would have an XPS binding energy of Si 2p around ~103.5 eV. Therefore, this rules out the possibility. Then, the other possible source is the non-attachment of 1 or 2 of the methoxysilanes. The XPS C1s signature shifted to higher binding energy. The shift could be a signature of a C-O chemical state. Both binding energies of the C1s peak and shoulder are assigned for adventitious carbon contamination in the literature. Thus, it would be difficult to differentiate the contamination from the –C=CH2 surface. However, our SFG spectra verified most silanol groups were converted to vinyl because of the =CH2 and C=C peaks in Figure 1. The C=O peak was too close to be 32


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resolved from the assigned C1s peak. The quantification of the organic functionalization of the surface was challenging for the conversion of –C=CH2 from TGA (-COOH) via thiol-ene reaction and -COOH to -COCl by acyl chloride formation. Therefore, no approximate conversion is provided. At the NPs attachment, no XPS signature was obtained for N, Fe, and Mg at this step to verify the attachment of the NPs. There are some factors that can lead to this result which are as follow: 1) sparse distribution of NPs under the probed area due to partial conversion to -COCl and ineffective coupling reaction of R-NH2 (decorated NPs) with R’-COCl (modified silica surface) and 2) the XPS signal was limited by its resolution and the amount of NPs. However, the NPs attachment was verified by acquiring an AFM image (Figure S16) of a decorated substrate after exposure to lead solution available in the SI. Next, XPS characterization was also performed for surfaces exposed to 0.03M and 300 pM lead solution. It was verified we could only detect the Pb 4f7/2 XPS signal at 139.2 eV at a higher concentration of lead at 0.03 M After calibration, the peak was assigned to a PbO signature. The PbO nature could come from the cation replacement of the surface H+ by the solution species (Pb2+) coupled with the coordinative bond formation of (-OH)2Pb2+ in coexistence with (-NH2)2Pb2+ and H+.14, 99, 100 at pD = 4.4. Using Rutherford backscattering spectroscopy (RBS), we were also able to quantify the amount of lead detected at the surface which was approximately 4 x 10 13 atoms/cm2 at the center of the substrate. The amount of lead varied across the sample from 8.5 x 10 12 atoms/cm2 to 5.7 x 1015 atoms/cm2. No iron was detected on the sample. The detection limit for iron was ~ 4 x 1013atoms/cm2. Precautions were taken during the analysis to determine that the lead content of the sample did not change during analysis. This was done by progressing increasing the exposure of the sample to the ion beam from 0.5 µC over the beam spot size of 1 x 2 mm2 to 20 µC. 33


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Within the statistical counting limit, no variation in the lead was detected. It might be expected that the ion beam would cause organic materials to be desorbed from the surface, which may have happened, but even if it did, it had no effect on the lead content. In addition, since no iron signal, was observed, but the lead was quantified. There is a possibility that RBS probing area did not contain sparsely enough distributed magnetic nanoparticles for iron to be detected within the limit of detection. However, the lead was measured in the same probing area. Therefore, the possible scenario would be the attachment of Pb2+ ion to -COOH terminated silica substrate even without nanoparticles. It was found that thioglycolic acid that is physically and chemically bound to a silica surface can be used to remove heavy metal ions from water samples including lead.101 At pH=4.65, the metal uptake capacity was found to be 0.503. Since the lead solution was prepared at pD = 4.4, it is probable for the Pb2+ to also adsorb on unconverted -COOH groups. With these possibilities, the adsorption capability of these decorated magnetic nanoparticles will need to undergo additional assessments to demonstrate its success as adsorbents for heavy metal adsorbates. In this work, magnetic MgFe2O4 NPs, as a model system of ferrite-based materials, were attached to a silica substrate with the help of a surface thiol-ene reaction. CONCLUSION We designed the surface preparation of a hydrophilic silica substrate by employing successive surface reactions that could be applied to the monitoring of biomolecules, macromolecules, and nanoparticles using SFG spectroscopy at the solid-air interface. The vinyl peaks at ~2986 cm-1 were used as an indicator to observe the surface functionalization of silica using vinyltrimethoxysilane (first step). Moreover, unreacted methoxy (-OCH3) groups were also 34


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found in the SFG spectra of the vinyl-terminated substrate. However, after converting the vinyl groups to carboxyl groups and employing the thiol-ene reaction as the second step, the vinyl peaks had significantly disappeared, indicating that the conversion had approached completion. To convert carboxyl-terminated groups to acyl chloride-terminated groups, thionyl chloride was employed to accomplish the necessary surface modification. The SFG spectra in the C-H region were the same for both cases where a shift in the carbonyl region was found due to the changing environment experienced by the carbonyl group in the shift from -OH to -Cl. Finally, the aminefunctionalized magnesium ferrite nanoparticles were attached to the silica substrate via amide bond formation after reacting with the acyl chloride-terminated silica substrate. We exposed the modified substrate with NPs to the Pb2+ solution and characterized the sample, at the solid-liquid interface. Changes in the SFG spectra after adding Pb2+ to the pure solvent were not apparent due to the low SFG signal generated by sparse attachment of the magnetic nanoparticles onto the modified silica substrate. To improve signal detection, the number of modified magnetic NPs attached to the substrate needs to increase, which can also consequently enhance the amount of SFG photons. However, the use of this modified substrate as a result of successive reactions is promising for the further study of surface adsorption by other adsorbates. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Reaction vessel, SFG setup, schematic diagrams for solid-air and solid-liquid interfaces, resonant SFG spectra, 1H NMR spectra, and additional SFG spectra. (PDF) 35


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AUTHOR INFORMATION Corresponding Authors: *Email: [email protected]. Tel.: +1-740-593-2308. Fax: +1-740-593-0148. *Email: [email protected]. Tel.: +66-53-943-341 ext. 204. Fax: +66-53-892-277. ORCID Katherine Leslee Asetre Cimatu: 0000-0002-4216-9715 Author Contributions *These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank the National Science Foundation under Grant Nos. CHE-0947031 and CHE1338000 for the acquisition of the femtosecond laser and nuclear magnetic spectrometer. The current work was also supported by the start-up fund provided by Ohio University. Additionally, the authors would like to thank the Nanoscale and Quantum Phenomena Institute and Condensed Matter and Surface Science for their additional financial support. The financial support for the preparation of nanoparticles was from the Center of Excellence in Materials Science and Technology, Chiang Mai University, under the administration of the Materials Science Research Center, Faculty of Science, Chiang Mai University. Jeeranan Nonkumwong would like to acknowledge the Science Achievement Scholarship of Thailand (SAST) for financial support for 36


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short-term research in the Cimatu research group, Department of Chemistry and Biochemistry, Ohio University. REFERENCES 1. Gopinath, S. C. B.; Awazu, K.; Fujimaki, M.; Shimizu, K.; Mizutani, W.; Tsukagoshi, K. Surface Functionalization Chemistries on Highly Sensitive Silica-Based Sensor Chips. Analyst 2012, 137 (15), 3520-3527. 2. Righini, G. C.; Chiappini, A. Glass Optical Waveguides: A Review of Fabrication Techniques. Optical Engineering 2014, 53 (7). 3. Kumar, A.; Kashyap, K.; Hou, M. T.; Yeh, J. A. Strength Improvement of Glass Substrates by Using Surface Nanostructures. Nanoscale Research Letters 2016, 11. 4. Hashimoto, S.; Uwada, T.; Hagiri, M.; Takai, H.; Ueki, T. Gold Nanoparticle-Assisted Laser Surface Modification of Borosilicate Glass Substrates. Journal of Physical Chemistry C 2009, 113 (48), 20640-20647. 5. Chen, E. H.; Walter, S. R.; Nguyen, S. T.; Geiger, F. M. Arylsilanated SiOx Surfaces for Mild and Simple Two-Step Click Functionalization with Small Molecules and Oligonucleotides. Journal of Physical Chemistry C 2012, 116 (37), 19886-19892. 6. Tucker-Schwartz, A. K.; Farrell, R. A.; Garrell, R. L. Thiol-ene Click Reaction as a General Route to Functional Trialkoxysilanes for Surface Coating Applications. Journal of the American Chemical Society 2011, 133 (29), 11026-11029. 7. Hoyle, C. E.; Bowman, C. N. Thiol-Ene Click Chemistry. Angewandte ChemieInternational Edition 2010, 49 (9), 1540-1573. 8. Xu, J. T.; Boyer, C. Visible Light Photocatalytic Thiol-Ene Reaction: An Elegant Approach for Fast Polymer Postfunctionalization and Step-Growth Polymerization. Macromolecules 2015, 48 (3), 520-529. 9. Bordoni, A. V.; Lombardo, M. V.; Regazzoni, A. E.; Soler-Illia, G.; Wolosiuk, A. Simple Thiol-Ene Click Chemistry Modification of SBA-15 Silica Pores with Carboxylic Acids. Journal of Colloid and Interface Science 2015, 450, 316-324. Wendeln, C.; Rinnen, S.; Schulz, C.; Arlinghaus, H. F.; Ravoo, B. J. Photochemical 10. Microcontact Printing by Thiol-Ene and Thiol-Yne Click Chemistry. Langmuir 2010, 26 (20), 15966-15971. 11. Lowe, A. B. Thiol-Ene "Click" Reactions and Recent Applications in Polymer and Materials Synthesis. Polymer Chemistry 2010, 1 (1), 17-36. 12. Laaniste, A.; Marechal, A.; El-Debs, R.; Randon, J.; Dugas, V.; Demesmay, C. "ThiolEne" Photoclick Chemistry as a Rapid and Localizable Functionalization Pathway for Silica Capillary Monolithic Columns. Journal of Chromatography A 2014, 1355, 296-300. 13. Bordoni, A. V.; Lombardo, M. V.; Wolosiuk, A. Photochemical Radical Thiol-Ene ClickBased Methodologies for Silica and Transition Metal Oxides Materials Chemical Modification: A Mini-Review. RSC Advances 2016, 6 (81), 77410-77426. Nonkumwong, J.; Ananta, S.; Srisombat, L. Effective Removal of Lead(II) from 14. Wastewater by Amine-Functionalized Magnesium Ferrite Nanoparticles. RSC Advances 2016, 6 (53), 47382-47393. 37


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Successive Surface Reactions on Hydrophilic Silica for Modified

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