Sum Frequency Generation Vibrational Spectroscopy Using

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Sum Frequency Generation Vibrational Spectroscopy using Evanescent Waves - towards Probing Irregular and Complex Surfaces of Mesoscopic-scale Materials Pengcheng Hu, Bolin Li, Chen Bai, Xu Li, and Xiaolin Lu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b03088 • Publication Date (Web): 16 Nov 2018 Downloaded from http://pubs.acs.org on November 17, 2018

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Analytical Chemistry

Sum Frequency Generation Vibrational Spectroscopy using Evanescent Waves - towards Probing Irregular and Complex Surfaces of Mesoscopic-scale Materials Pengcheng Hu,† Bolin Li,† Chen Bai,† Xu Li,† Xiaolin Lu*,† †State

Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Nanjing 210096, China ABSTRACT: With rapid development of materials science, on-demand techniques are highly needed with the capability to characterize materials in the micrometer and nanometer scales. In this article, we show that, by employing a prism geometry, total internal reflection (TIR) sum frequency generation (SFG) vibrational spectroscopy allows for characterizing mesoscopic materials with irregular or complex surfaces. Four representative examples were presented. First, we reveal that, mechanical grinding can subtly alter the surface molecular structures of original materials. Second, spin coating can substantially change the surface molecular structures of particle samples. Third, surface restructuring of carboxylated multi-walled carbon nanotubes (c-MWCNTs) can happen in response to the surrounding environment. Fourth, surface adsorption and desorption dynamics of toluene on activated charcoal can be traced. Such experiments demonstrate, there are still a broad range of research fields ahead SFG can be directed to, where materials in mesoscopic scales with irregular or complex surfaces can be studied.

INTRODUCTION Nowadays, with the advancement of materials science and engineering, a huge number of materials in the mesoscopic scales (micrometer (m) or nanometer (nm)) are being fabricated with the designed chemical, physical, optical, electrical or/and biological function.1-3 In consequence, twofold aspects have to be considered for such small sized materials. On one side, the micrometer or nanometer scale poses the microscopic geometric confinement to such materials, leading to the size effect on various properties. On the other side, with the diminished length scale, the exponentially increased specific surface area can render nonnegligible or even decisive influence on the performance of such materials. From the viewpoint of the fundamental science, it is pivotal to understand the molecular-level surface structures of such mesoscopic materials and build up the necessary connection to their functions, in order to accurately and effectively make full use of such materials. In this context, it is of great importance to revisit the local molecular-level surface structures of such small sized materials in comparison to those of the macroscopic ones. A series of characterization techniques, including attenuated total reflection infrared spectroscopy (ATR-IR), surface enhanced raman scattering (SERS), x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), etc.4-9 have been used to characterize such mesoscopic materials to obtain structural information like surface species, surface compositions, and surface morphologies, etc. Each characterization technique has its own advantage(s) over others. However, owing to the intrinsic limitation or restriction of such techniques, e.g. either suffering from the surface selectivity (the interference from the bulk) or the requirement of the high vacuum, the technological improvement or certain

new approach is highly needed. Regarding to this, the liquidphase electron microscopy10,11 and super-resolution optical microscopy12,13 developed in recent years provide two excellent examples, in which new methodological approaches were constructed to break through the limitations of the old ones. Sum frequency generation (SFG) vibrational spectroscopy, as a second order nonlinear optical technique developed for more than thirty years since the first series of papers were published in 1987 by Shen et al.,14-16 has shown significant advantages over the traditional ones due to its intrinsic surface selectivity and molecular orientation sensitivity. Surface and interfacial molecular-level structures for a wide range of materials have successfully been studied,17-22 providing new knowledges and promoting our understanding on the materials’ surfaces and interfaces. It should be noted, as a coherent process, interaction of the two incident beams with molecules at a flat surface or interface leads to a third sum frequency beam with the defined output direction. This predestined condition renders the SFG vibrational spectroscopy suitable for detecting flat surfaces/interfaces. For materials with nonflat (irregular or complex) surfaces, the generated SFG signals will be scattered, posing a serious problem on the signal detection. For example, to detect the surface molecular structures of particle samples, a spherical mirror could be mounted to collect the diffuse reflected SFG signals generated from the particle surfaces, as demonstrated by Allen et al.23 In exchange, the labor for mounting an additional mirror and succedent optic setup made it difficult to be a standard and generalized methodology towards studying materials with irregular or complex surfaces. Regarding the total internal reflection (TIR) geometry for SFG and difference frequency generation (DFG), the order-of-magnitude enhanced nonlinear response was predicted by Dick, et al.24 In 2000, a

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second harmonic generation scattering method was brought out by Wang et al. to trace the surface adsorption process on talc particles in water.25 In 2006, Yeganeh et al. reported their investigation of the adsorption and desorption dynamics of methanol on the high-surface-area silicon dioxide (SiO2) powdered samples, where the total internal reflection geometry was successfully employed.26 It is a pity that extension of this work was not reported afterwards. Recently, Roke et al. developed an SFG scattering method to study the surfaces of the dispersed clusters, droplets and particles in the liquid environments,27-29 which represents a significant progress in this field. However, similar to the case of the diffuse reflected SFG, the additional optical setup and data interpretation may impede the generalization and application of this scattering method. In this respect, a simple and convenient experimental setup or approach had better be employed. Herein, inspired by the above-mentioned studies,2329 we are using the SFG vibrational spectroscopy with a prism experimental setup to study several representative mesoscopic materials. The evanescent waves generated by this total internal reflection (TIR) experimental setup allows us to probe irregular or complex surfaces. The main text of this paper will be divided into four parts. First, we show that, the surface molecular structures of several commercially available powdered polymer samples can subtly be changed by the mechanical grinding, a commonly used method to acquire fine powdered materials. Second, the generally used dissolving and spin-coating method can substantially change the surface molecular structures with respect to the original ones. Third, the surface functional group restructuring of the carboxylated multi-walled carbon nanotubes (c-MWCNTs) can easily be detected by SFG in response to the surrounding hydrophobic/hydrophilic environments. Fourth, by detecting the vibrational signals of the toluene molecules adsorbed onto and desorbed from the activated charcoal, we show that the surface dynamic processes can be traced via SFG.

EXPERIMENTAL SECTION Materials. Powdered poly(methyl methacrylate) (PMMA, Mw=~120,000), poly(benzyl methacrylate) (PBenMA, Mw=~100,000), polystyrene (PS, Mw=~48100) and decahydronaphthalene (decalin, C10H18) were purchased from Sigma Aldrich. Polyethylene glycol (PEO, Mn=~20,000) and activated charcoal (average particle size is ~75 m) were purchased from Aladdin Co., Ltd. Multi-walled carbon nanotubes (MWCNTs, diameter =8 nm) and carboxylated multi-walled carbon nanotubes (c-MWCNTs, diameter =8 nm, and -COOH content is 3.86%) were purchased from Chengdu Organic Chemical Co., Ltd. The iron oxide (Fe3O4, =20 nm) nanoparticles decorated with a liposome (DSPEPEG2000-COOH, 1,2-Distearoyl-sn-glycero-3phosphoethanolamine-N-carboxy(polyethylene glycol)) surface was synthesized in our laboratory (Fe3O4@DSPEPEG2000-COOH, see Supporting Information for detail). Other daily used samples were ordered from the supermarket, including the powdered wheat flour, powdered cookies (Mondelez International Co., Ltd) and milk powders (Mongolia Yili Industrial Co., Ltd). Isosceles right-angle calcium fluoride (CaF2) prisms and CaF2 windows were purchased from Chengdu YaSi Optoelectronics Co., Ltd. Deionized water (18.2 MΩ·cm) was acquired from a Milli-Q synthesis system (Millipore, Billerica, MA). The detailed description of experimental pretreatment (prism cleaning

process) and samples preparation are shown in supporting information. All the chemicals were used without further purification. Attention should be paid, as an order-ofmagnitude estimation, the surface roughnesses for the powdered polymer samples (PMMA, PBenMA, PS, and PEO) and active charcoal are of the micrometer scales; the surface roughnesses for the MWCNTs and c-MWCNTs are of the nanometer scales; and the surface roughnesses for powdered cookies, powdered wheat flour, milk powders and Fe3O4@DSPE-PEG2000-COOH particles are of the micrometer scales. The latter SEM images will show the morphologies of these samples. SFG Experimental Methodology. It has been more than thirty years since the first series of papers on the SFG vibrational spectroscopy were published.14-16 Significant advancement was achieved on account of the theory, experimental setup and applications.17,30-36 Currently, there are two widely employed types of SFG systems, i.e. IR frequency scanning system and broadband IR system.30-38 In this paper, the IR frequency scanning system (Pico-second SFG system, EKSPLA, Lithuania) with the co-propagation mode was employed.

Scheme 1. (a) Schematic shows our experimental setup using a CaF2 prism with its bottom plane in contact with microsized or nanosized samples. (b) The incoming IR and visible beams pass through the prism and evanescent waves are generated below the prism, which interact with the surfaces of microsized or nanosized samples. Part of the scattered sum frequency beam is collected. This total internal reflection (TIR) allows us to collect scattered sum frequency signals with enough intensity for further analysis. The prism SFG experimental setup was shown in Scheme 1, the detailed process was that the powdered samples were put into a small open vessel (height, 50 mm; diameter, 60 mm). The top of the powdered samples was higher than the top edge of the vessel. Then the vessel was put into contact with the prism and fixed using the adhesive tape. A frequency-tunable infrared (IR, ωIR) beam and a frequency fixed visible (VIS, ωVIS, =532 nm) beam propagated across the transparent CaF2 prism, temporally and spatially overlapped at the prism bottom plane, below which the target microsized or nanosized sample was placed. The diameters for both the IR and visible beams were ~0.5 mm. The pulse energies for the IR and visible beams were ~70 μJ and ~45 μJ respectively, which were measured by a digital power & energy meter (Coherent, Inc.). When the incident angles for the visible and IR beams are appropriate, i.e. 60° and 55° respectively in this study, total internal reflection (TIR) will happen. To further demonstrate the advantage of the TIR geometry, the SFG spectra for unground sample particles with the normal reflection mode were also collected as a control, in which the surface


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Analytical Chemistry vibrational signals for the sample particles can hardly been differentiated, as shown in Figure 1 and Figure 2 (bottom ones). The TIR geometry has been widely employed in infrared spectroscopy and fluorescence imaging system. Extension of the TIR geometry to SFG is of technologically significance. Using the TIR geometry, the Fresnel coefficients (local field coefficients, the calculated results were shown in Table S2 and Figure S5) can greatly be enhanced, leading to the feasibly detectable SFG signals. For the irregular and complex surfaces, using the TIR geometry, more scattered wave vectors become detectable in comparison to the normal reflective mode. Therefore, the detection efficiency of the scattered signals is largely improved, which is beneficial for studying the rough surfaces. The exponentially decayed evanescent waves below the prism bottom plane can interact with the bottom sample particles provided that the sample particle surfaces were close enough (while there still existed the air gap between them), namely, within the penetration length (Lps) of the evanescent waves, as demonstrated by Yeganeh et al.26 Using Equation 1, the estimated Lps for the IR and visible beams are ~1000 nm and ~200 nm respectively. In this case, the particle surfaces must be placed within a length scale of ~200 nm from the prism bottom plane. By satisfying the above conditions, the sum frequency (ωSUM=ωIR+ωVIS) signals from the particle surfaces were generated. Although the generated SFG signals were scattered, owing to this TIR setup, the collected part of the SFG signals was still strong enough to be differentiated by our detector (monochromator/photomultiplier tube).

Lp s 





2n1 sin 2   (ns / n1 ) 2

Panel i and Panel i' of Figure 1 show the SFG spectra of the PEO sample particles. Similar SFG spectra were collected before and after the mechanic grinding for both ssp and ppp polarization combinations, i.e. a strong C-H symmetric stretching (ss) peak at ~2870 cm-1, a weak anti-symmetric stretching (as) peak at ~2908 cm-1 and a weak Fermi resonance (Fermi) peak at ~2945 cm-1 for the CH2 groups.39-41 The similar SFG spectra for PEO before and after the mechanic grinding indicate that the surface molecular-level structures were similar. We thus assume the reason may be correlated to the PEO chain configuration, namely, PEO is a polymer without the side group. To prove whether this argument is right or wrong, the surface SFG spectra for PMMA and PBenMA were also collected, as shown in Panel ii, and Panel ii' of Figure 1. For PMMA, both the ssp and ppp spectra before and after the mechanic grinding were similar. The two strongest peaks at ~2880 cm-1 and ~2935 cm-1 could sequentially be assigned to the ss mode and Fermi resonance



1/ 2

(1) In Equation 1, Lps represents the depth of the light penetration; λ and α represent the wavelength and angle of the incidence; n1 represents the refractive index of the CaF2 prism; ns represents the effective refractive index of the bottom medium (averaged one for the air and the samples). It should be emphasized, owing to the rapid attenuation (exponentially decayed) of the light fields, the generated SFG signals were mainly from the interfacial area between the prism and the sample, where the irregular surfaces of our samples were also located, rather than from all the surfaces over the overlapped penetration length of ~102 nm for both the infrared and visible beams. In this study, two polarization combinations, i.e. ssp (spolarized sum frequency, s-polarized visible, and p-polarized infrared beams) and ppp, were used. Each SFG spectrum was an averaged one from three different measurements.

RESULTS AND DISCUSSION Effect of Mechanic Grinding on Polymer Surface Structures. As shown in Figure 1, the SEM images (inserted, scale bar - 200 m) disclose that the particle sizes for all the three samples (PEO, PMMA, and PBenMA) did decrease after the mechanic grinding. But the zoom-in images (scale bar - 2 m) didn’t show any substantial change for the surface morphologies before and after the mechanic grinding. Albeit, the surface molecular-level structures need to be detected for further inspection. It should be noted, configurationally, PEO is a polymer without the side group while PMMA and PBenMA are two polymers with the side groups.

Figure 1. SEM images and SFG (ppp and ssp combinations, Panels i and i', Panels ii and ii', Panels iii and iii' are for PEO, PMMA and PBenMA particles, respectively) spectra for polymers before and after mechanic grinding. A, B and C are the SEM images before mechanic grinding for PEO, PMMA and PBenMA respectively; a, b and c are the corresponding SEM images after mechanic grinding. Inserted are the zoomout images. “Control_NR” corresponds to the normal reflective mode.



of the -methyl group or vice versa. The weak peaks at ~2855 cm-1, ~2905 cm-1 and ~2990 cm-1 can sequentially assigned to the ss mode of methylene, Fermi resonance of methylene and as mode of the -methyl group.17,30,31,42-44 It could thus be concluded, for the PMMA sample particles, both before and after the mechanic grinding, the particle surfaces are dominated by the -methyl group. For PBenMA, as shown in Panel iii and Panel iii' of Figure 1, the first significant difference before and after the mechanic grinding is the spectral intensity, i.e. the spectral intensity decreased after the mechanic grinding for both the ssp and ppp polarization combinations. The second one is the relative decrease of the backbone C-H signals (~2800 cm-1 to ~3000 cm-1) in comparison to the phenyl C-H signals (~3000 cm-1 to ~3100 cm-1). The backbone C-H vibrational peaks include the ss mode of methylene at ~2850 cm-1, the ss mode/Fermi resonance of the -methyl group at ~2880 cm-1 and ~2935 cm1 (similar to the case for PMMA), the Fermi resonance of methylene and as mode of the -methyl group at 2901 cm-1 and ~2965 cm-1, respectively. The phenyl C-H vibrational peaks include combination band mode at ~3005 cm-1, 20b mode at ~3020 cm-1, 7a mode at ~3037 cm-1, 7b mode at ~3052 cm-1, and 2 mode at ~3068 cm-1.45 Such SFG experimental results indicate, first, the surfaces of the PBenMA particles before the mechanic grinding were more ordered than those after the mechanic grinding, especially for the -methyl groups; second, after the mechanic grinding, the side phenyl groups became relatively more ordered than the methyl groups.

Figure 2. The SFG (ppp and ssp) spectra of different polymers particles (before and after mechanic grinding) in the C=O range. Panels a and b are the SFG spectra of the PMMA in air. Panels c and d are the PBenMA in air. “Control_NR” corresponds to the normal reflective mode. For PMMA and PBenMA, since the carbonyl groups (C=O) are linked to the side methyl groups (PMMA) or phenyl groups (PBenMA), the vibrational signals in the C=O range were also collected, as shown in Figure 2. For PMMA, the C=O vibrational signals before and after the mechanic grinding are similar for both the ssp and ppp spectra, showing the consistence with those in the CH frequency range. But for PBenMA, in the ppp spectrum, a C=O stretching vibrational peak at ~1730 cm-1 was observed before the mechanic grinding.30,44,46,47 After the mechanic grinding, this peak became weaker. While for ssp, the C=O vibrational peak

became stronger after the mechanic grinding. This suggests the reorientation of the C=O groups happened during the mechanic grinding process. Therefore, unlike PEO and PMMA, the mechanic grinding did affect the PBenMA particle surfaces by changing the surface order of the polar functional groups to certain extent. The above SFG experiments clearly demonstrate, the surfaces of the materials in the mesoscopic scales (m and nm) with irregular or complex shapes can be detected by TIRSFG. As is known, for a polymer surface, its physical/chemical properties are essentially decided by its monomer units, including the backbone and side groups, and how these groups arrange themselves at the surface. Therefore, this TIR-SFG method opens a door for characterizing the fine powdered polymer samples with irregular or complex surfaces. Particle and Flat Film Surfaces. In the industry and academy, the spin coating is a general method to prepare the polymer thin films with flat and smooth surfaces. Intuitively, such surfaces shouldn’t be the same as those of the particle samples. However, to the best of our knowledge, no experimental study has been involved in this issue, i.e. to compare the structural difference between the polymer particle surfaces and flat film surfaces. Herein, we reveal, the molecular-level surface structures of the flat films are substantially different from those of the particle samples. Three sample polymers were investigated in this study including PS, PMMA and PBenMA. For the particle samples, the TIR geometry (CaF2 prism) was employed to collect the SFG spectra. For the film samples, the external reflection (ER) geometry (CaF2 window) was employed. The collected SFG spectra were shown in Figure 3. For the PS particle samples (Panels i and i' of Figure 3), whether for the ssp or ppp polarization combination, the C-H stretching vibrational signals in the frequency range from 2800 cm-1 to ~3000 cm-1 were much stronger than those in the frequency range from 3000 cm-1 to ~3100 cm-1 (phenyl C-H stretching range), including the ss mode of methylene at ~2855 cm-1, the ss mode of the end methyl group at ~2875 cm-1, the methyne (CH) stretching mode at ~2913 cm-1, Fermi resonance at the ~2935 cm-1 (may overlap with the as mode of methylene), and the as mode of the end methyl group at ~2962 cm-1. For the PS film samples prepared via the spin coating, the phenyl C-H stretching vibrational signals are much stronger than those in the frequency range from ~2800 cm-1 to ~3000 cm-1. And, in the ppp spectrum, there were two prominent peaks located at ~3032 cm-1 and ~3063 cm-1 which are sequentially assigned to the 7a mode and 2 mode. In the ssp spectrum, the prominent peak was the 2 mode at ~3063 cm-1.30,31,45,48-50 The above experimental results indicate, at the surfaces of the PS particle samples, the end methyl and the backbone methylene groups are polar ordered; while at the surface of the PS film, the side phenyl groups are polar ordered. For the PMMA particle samples, as shown in Panels ii and ii' of Figure 3 and Panels ii and ii' of Figure 2, the two dominated peaks were the ss mode and Fermi resonance of the -methyl group at ~2880 cm-1 and ~2936 cm-1 (or vice versa). But for the film sample, in the ssp spectrum, the dominated peak was the ss mode of the ester methyl group at ~2954 cm-1; and in the ppp spectrum, the two dominated peaks were the as mode of the ester methyl group at ~2988 cm-1 and 3020 cm-


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Analytical Chemistry 1.17,30,31,42,43,44,51

It can be concluded, at the PMMA particle surfaces, the -methyl groups are polar ordered; while at the PMMA film surface prepared via the spin coating, the ester methyl groups are polar ordered. For the PBenMA particle samples, as shown in Panels iii and iii' of Figure 3 and Panels iii and iii' of Figure 2, the dominated peaks include the ss mode of the -methyl group at ~2880 cm-1, the as mode at ~2935 cm-1 and the 2 mode at ~3068 cm-1, for both the ssp and ppp spectra. For the PBenMA film sample, in the ssp spectrum, the dominated peak was the 2 mode at ~3068 cm-1; while in the ppp spectrum, the dominated peak was the 7a mode at ~3033 cm-1. Such spectral characteristics strongly suggest, at the PBenMA particle surfaces, the side -methyl groups and the backbone methylene groups rather than the side phenyl groups are polar ordered; while at the PBenMA film surface, the side phenyl groups rather than the side methyl groups and the backbone methylene groups are polar ordered.

Figure 3. The SFG (ppp and ssp) spectra for the particle and film samples of PS (Panels i and i'), PMMA (Panels ii and ii') and PBenMA (Panels iii and iii'). The corresponding schematic shows the local molecular-level structures at the particle and film surfaces for PS (a and a'), PMMA (b and b') and PBenMA (c and c') (Panels a, b and c are schematics for polymer particle samples; Panels a', b' and c' are schematics of the polymer film samples spin-coated on the CaF2 window substrates).

Further evidence was shown in Figure 4, where the vibrational signals in the C=O range for PMMA and PBenMA were presented.30,44,46,47 For the PMMA particle samples, the

Figure 4. The SFG (ppp and ssp) spectra in the C=O range for the particle and film samples of PMMA (Panels a and b) and PBenMA (Panels c and d). C=O vibrational peak in the ppp spectrum was much stronger than that in the ssp spectrum. But for the PMMA film sample, the C=O vibrational peak in the ssp spectrum was much stronger than that in the ppp spectrum. Similar to PMMA, for the PBenMA particle and film samples, the same trend was found, i.e. the C=O vibrational peak in the ppp spectrum was much stronger than that in the ssp spectrum for the particle sample while the C=O vibrational peak in the ssp spectrum was much stronger than that in the ppp spectrum for the film sample. Such SFG spectra in the C=O range again support that the side ester methyl or the side ester benzyl groups were highly polar ordered. As shown in the schematics of Figure 3, the possible local molecular-level structures at the particle and film surfaces for PS, PMMA and PBenMA were presented. Surface Functional Group Restructuring of c-MWCNTs. Owing to the excellent chemical, electrical, mechanical, and even optical properties, the carbon nanotubes (CNTs) have widely been used in the past few decades.52,53 Many characterization techniques, including FTIR, Raman spectroscopy, and transmission electron microscopy (TEM) etc. have been employed to characterize the CNTs. In this section, MWCNTs and c-MWCNTs were chosen for the TIRSFG experiment. Figure 5 shows the SFG spectra of MWCNTs and c-MWCNTs in different states, i.e. in air, wetted by decalin and wetted by water. For MWCNTs, since there is no functionalized carboxylated group, the C=O stretching vibrational signal was not detected for all the samples in the above-mentioned three states. For c-MWCNTs, in the hydrophobic environment, namely in air or wetted by decalin, no C=O stretching vibrational signals were observed, either. However, for c-MWCNTs wetted by water, a significant C=O stretching peak at ~1725 cm-1 was observed for both the ssp and ppp spectra. Such experimental results indicate, in the hydrophobic environment, the carboxylated functional groups at the c-MWCNTs surfaces would be randomly distributed with respect to the orientation; while in the hydrophilic environment, the carboxylated functional groups would protrude out of the c-MWCNTs surfaces to adapt to the outside environment. It should be noted, such surface restructuring information cannot be obtained with the



traditional vibrational spectroscopies like FTIR and Raman spectroscopy. (FTIR and Raman spectra for the MWCNTs and c-MWCNTs in the Supporting Information, Figure S1)

Figure 5. The SFG (ppp and ssp) spectra of c-MWCNTs (Panels a and b) and MWCNTs (Panels c and d) in the C=O stretching range in different states, namely in air (bottom curves), wetted by decalin (C10H18, middle curves) and wetted by water (top curves). Adsorption and Desorption of Toluene in Activated Charcoal. Owing to its large surface-volume ratio and excellent chemical stability, activated charcoal has widely been used as an efficient adsorbent. Examples include gas storage (SOx adsorption, CO2 capture, H2S removal, and H2/CH4 storage), wastewater treatment (dye, pigment or heavy metal ions removal) and energy storage (electrode materials or solid-state hydrogen stores)54-56. In this section, the adsorption and desorption processes of toluene by activated charcoal were traced by TIR-SFG. The activated charcoal particles we used are highly irregular and there is a broad distribution for the particle sizes from nanometers to micrometers, as shown in Figure 6 (Panels a and b) and Figure S2 (SEM images with different magnifications). (see Supporting Information) The pure activated charcoal didn’t present any SFG vibrational signal, as shown in Panels e and f of Figure 6. After the adsorption (~3 hours’ exposure to toluene vapor), The toluene characteristic bands were observed, including the 2 mode at ~3055 cm-1, the as mode of the methyl group at ~2924 cm-1 and the ss mode at ~2872 cm-1, as shown in Panels e and f of Figure 6.57,58 A control adsorption experiment was also run without the activated charcoal, which proved that the toluene molecules cannot be adsorbed onto the bare CaF2 prism surface (see Supporting Information). To trace the surface adsorption or desorption dynamics, the time-dependent SFG intensity variation of the toluene 2 mode at ~3055 cm-1 was monitored with the ppp polarization combination at the room temperature (~221 C). As shown in Panel c of Figure 6, in the adsorption process, the sample holder was exposed to the toluene vapor at ~400 s. But the SFG intensity was starting to uplift at ~3000 s. This reflects the period of time necessary for the toluene molecules to diffuse to the activated charcoal particles attached to the CaF2 prism bottom. After ~3000 s, the vibrational signal of the 2 mode started to increase. The adsorption curve was fitted with an empirical formula (Equation S1 in Supporting Information and Table S1), which leads to an apparent adsorption coefficient ka = 4.3×10-4 and a

dimensionless saturated adsorption value I,a = 0.44. Subsequently, the desorption process was monitored at the elevated temperatures (~855 C), as shown in Panel d of Figure 6. Right after the activated charcoal was heated up at ~1100 s, the vibrational signal at ~3055 cm-1 started to decrease and leveled off at ~5000 s. Analogously, the desorption curve can be fitted with an empirical desorption formula (Equation S2 and Table S1), leading to an apparent desorption coefficient kd = 4.8×10-4 and a dimensionless desorption value I,d = 0.17. As shown in Panels e and f of Figure 6, the SFG spectra indicate, after the desorption, there

Figure 6. Schematics show the adsorption (a) and desorption (b) of the toluene molecules in activated charcoal. (c) Dynamic adsorption curve by tracing the vibrational intensity change at ~3055 cm-1; activated charcoal was exposed to toluene at ~400 s. (d) Dynamic desorption curve by tracing the vibrational intensity change at ~3055 cm-1; activated charcoal was heated up at ~1100 s (~855 °C). (e) and (f) are the static SFG spectra for the activated charcoal in different states, i.e. original, after adsorption and after desorption. were still some toluene molecules in the activated charcoal. So the vibrational signal at ~3055 cm-1 after the desorption didn’t reach its original value before the adsorption. The above quantitative study demonstrates that the TIR-SFG (using the evanescent waves) can be applied to study the surface dynamics of the mesoscopic (m or nm or both) materials with irregular or complex surfaces. In Panels a and b of Figure 6, schematics show the adsorption and desorption of the toluene molecules in the activated charcoal.

CONCLUSIONS In this article, we prove that, using the TIR-SFG with the evanescent waves, the surface molecular-level structures can be studied for the mesoscopic materials, i.e. materials in the micrometer and nanometer scales. To prove this, example studies were presented in detail. (i) A general method to obtain


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Analytical Chemistry fine powdered samples, i.e. mechanic grinding, cannot affect the surface molecular structures of polymers without the side group, but can somehow affect the surface molecular structures of polymers with the side group(s) (for example PBenMA). (ii) The surface molecular structures of the spincoated films are substantially different from those of the original particle samples. Regarding this point, the R&D scientists must pay great attention. Because of the ease of collecting the spectra, the polymer thin films have generally been prepared via spin coating. However, the surfaces of the spin-coated polymer thin films cannot reflect their original states of the particle samples before the industrial processing. In return, this reminds us of a basic principle in the polymer science, i.e. history defines the metastable state of a polymer. (iii) Restructuring of surface carboxylated c-MWCNTs was investigated in air, upon contact with decalin and water. Depending on the environmental hydrophobicity or hydrophilicity, the surface carboxyl groups can either be randomly distributed at the surface or protrude out of the surface, resulting in “yes” or “no” vibrational signal in the C=O range. (iv) The adsorption and desorption processes of the toluene molecules in the activated charcoal can quantitatively be studied by tracing the vibrational signal of the 2 mode from the toluene molecules. The above designed experiments demonstrate, using the evanescent waves, the TIR-SFG vibrational spectroscopy can be directed to a broad range of research fields, especially for mesoscopic materials with irregular or complex surfaces in the micrometer or nanometer scales or both. In the Supporting Information, the surface TIR-SFG spectra for more example mesoscopic materials are given, including powdered cookies, powdered wheat flour, milk powders and synthetic Fe3O4@DSPEPEG2000-COOH nano-drug. We believe, in the near future, the SFG vibrational spectroscopy will be employed to investigate a large number of mesoscopic materials with nonflat or complex surfaces and more exciting scientific findings will be reported.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available on the ACS Publications website. SEM images, FTIR and Raman spectra, and fitted parameters, such information is available free of charge on the ACS Publications website at DOI:.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (X.L)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21574020), the Fundamental Research Funds for the Central Universities, the National Demonstration Center for Experimental Biomedical Engineering

Education (Southeast University). and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). P. H. is also grateful for the Graduate Students’ Scientific Research Innovation Project of Jiangsu Province Ordinary University (KYLX16_0287).

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