Hyperspectral imaging of acetaminophen on multi-walled carbon

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Hyperspectral imaging of acetaminophen on multi-walled carbon nanotubes Yifei Wang, Wanyi Fu, Yuxiang Shen, Appala Raju Badireddy, Wen Zhang, and Haiou Huang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02939 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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Hyperspectral imaging microscopy of acetaminophen adsorbed on multi-walled carbon

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nanotubes Manuscript to be submitted to Langmuir 10/14/2018 Yifei Wanga,b,§, Wanyi Fuc,§, Yuxiang Shend, Appala Raju Badireddyd, Wen Zhangc, Haiou Huangb,*

3 4 5 6 7 8 9 10 11 12 13 14 15

a

National Engineering Laboratory for Advanced Municipal Wastewater Treatment and Reuse Technology, Key Laboratory of Beijing for Water Quality Science and Water Environment Recovery Engineering, Beijing University of Technology, Beijing 100124, China.

b

State Key Joint Laboratory of Environmental Simulation and Pollution Control, School of Environment, Beijing Normal University, No. 19, Xinjiekouwai Street, Beijing 100875, China.

c

Department of Civil and Environmental Engineering, New Jersey Institute of Technology, Newark, NJ 07029, USA.

d

Department of Civil and Environmental Engineering, University of Vermont, Burlington, Vermont 05405, United States.

*

Corresponding author. phone: +86 10 5880 7743. fax: +86 10 5880 7743. Email: [email protected] (H. Huang)

16

17

§ The

first and second authors contributed equally to this work.

18 19 20 21 22

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ABSTRACT

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In this study, enhanced dark-field hyperspectral imaging (ED-HSI) was employed to directly

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observe acetaminophen (AAP), a model pharmaceutical and personal care product (PPCP),

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adsorbed on multi-walled carbon nanotubes with large diameters (L-MWCNT) and small

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diameters (S-MWCNT) under equilibrium conditions. The ED-HSI results revealed that (1) AAP

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molecules primarily adsorbed onto the external surfaces, rather than the internal surfaces of L-

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and S-MWCNT aggregates, (2) or on sidewall of the dispersed tubes, but not at their end caps.

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Besides, ED-HSI images showed that surface coverage ratio of AAP/S-MWCNT is smaller than

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that of AAP/L-MWCNT (1.1 vs 3.4), indicating that there are more available adsorption sites on

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S-MWCNT than L-MWCNT when the adsorption reached equilibrium. This finding was

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consistent with the adsorption capacities of S-MWCNT and L-MWCNT (252.7 vs 54.6 mg g-1).

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Direct visualization of sorption sites for PPCP molecules provides new insights into the

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heterogeneous structures and surface properties of MWCNT and helps elucidate the adsorption

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mechanisms that are fundamental to the design of functional adsorbents for PPCP contaminants.

40 41

Keywords: Acetaminophen; Adsorption; Dark-field hyperspectral imaging; Multi-walled carbon

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nanotube; Atomic force microscopy-Raman.

43 44 45 46


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1. INTRODUCTION

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Pharmaceuticals and personal care products (PPCP) encompass diverse types of

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functionalities, such as antibiotics, supplements, drugs, and cosmetics. However, their

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widespread uses and continuous releases into the environment have caused growing concerns

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about their potential harms to ecological systems and human health.1-4 For example, Liu et al.

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found that consistent exposure to antibiotic and hormone drugs resulted in the emergence of

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resistant bacteria strains and increased human health risks.3 Shen et al. demonstrated that

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carcinogenic nitrosamines can be generated during chlorine disinfection of water containing

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PPCP.5 Therefore, various technologies such as advanced oxidation (H2O2/UV),6, 7 nanofiltration

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(NF)/reverse osmosis (RO),8 and adsorption (i.e. activated carbon, carbon nanotubes and

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graphene)9-11 have been applied for PPCP removals from water and wastewater. Among them,

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PPCP adsorption using carbon nanotubes (CNT) has drawn increasing attentions due to the

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unique structure and superb adsorption capacity possessed by CNT.12,

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understanding of PPCP adsorption by CNT is a prerequisite for the development of sustainable

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PPCP treatment techniques based upon CNT adsorption.

13

A mechanistic

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To date, a vast number of studies have been reported in literature pertaining to the adsorption

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mechanisms of PPCP on CNT. Lin and Xing inferred four possible interactions between PPCP

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and CNT, i.e., hydrophobic interaction, electrostatic interaction, hydrogen bonding interaction,

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and π-π bonds by analyzing static adsorption results obtained in varying conditions.14 By

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comparing the PPCP adsorption under different water chemical conditions, Li et al. studied the

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adsorption behaviors of ionizable pharmaceuticals including benzoic acid, phthalic acid, and 5 ACS Paragon Plus Environment

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2,6-dichloro-4-nitrophenol onto CNT adsorbents containing different types of oxygenated

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functional groups, and found that the adsorption was primarily attributed to the formation of a

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negative charge-assisted H-bond between a carboxyl group on the solute and a phenolate or

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carboxylate group on the surface of CNT.15 However, these studies did not directly observe the

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adsorbed PPCP molecules, neither specify the exact locations on CNT at which PPCP molecules

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adsorbed.

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In related studies conducted for contaminant adsorption onto CNT in aqueous solutions,

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heterogeneous adsorption was also observed..16-19 For example, Wi śniewski et al. combined the

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experimental and simulation results for the benzene adsorption onto a series of oxidized

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MWCNT, and found that significant effect of the surface heterogeneity had significant effects on

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the benzene adsorption enthalpy and entropy at low surface coverages.16 Angelikopoulos et al.

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used dynamics simulations model to analyze the surfactant adsorption behavior on bundle and

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individual CNT with small-diameter and discovered that the surface aggregation of surfactants

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appeared to influence the adsorbed amount significantly during adsorption process, which was a

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Langmuir-type process.17 Shen et al. declared that the sorbents of carbon nanomaterials with

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different oxygen content possessed strong sorption sites heterogeneity, which correlated with the

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sorption capacity of the chemical compounds of naphthalene, lindane, and atrazine by the

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sorbents.18 Similarly, Liu et al. coupled the well-known integral equation method and derivative

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isotherm summation (DIS) procedure based on a patchwise model to investigate (multi-walled

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carbon nanotube) MWCNT surface heterogeneity, and found it effective to understand surface

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heterogeneity.19 Dissipative particle dynamics simulations of a mesoscale model were performed 6 ACS Paragon Plus Environment

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to investigate the surfactant adsorption on small-diameter carbon nanotubes and their bundles.17

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In summary, the CNT heterogeneous adsorption mechanisms were mainly studied using model

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simulations and experiments results, which does not offer any information on the nature of

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association between the sorbate and sorbent in aqueous systems. Thus, there is a critical need for

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microscopy and spectroscopy tools which can accurately elucidate the mechanisms of

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association between sorbents and sorbates in aqueous systems (unlike traditional methods

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wherein analysis were done on dried samples.

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Enhanced dark-field microscope equipped with hyperspectral imaging (ED-HSI) is superior

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to the regular optical microscopes in the detection and characterization of engineered

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nanoparticles in environmental systems.20,

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By combining spectrophotometry and

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high-resolution imaging, ED-HSI collects spectra of reference materials with known components

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(e.g., pure AAP) and correlates the reference spectra with a target material (e.g., AAP/CNT

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mixture) to generate a hyperspectral image and identify specific components of interest in the

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aqueous samples.22,

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nanomaterials, such as silver, gold, and single walled carbon nanotubes (SWCNT).20,

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Besides, atomic force microscopy with Raman spectroscopy (AFM-Raman) has increasingly

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been used to study semiconductors, graphene, carbon nanotubes, polycrystals and epoxy

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compound.29-34 The integration of AFM with Raman spectrometry examines samples by a

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specific shuttle stage that allows transferring the sample from the AFM stage to the Raman

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microscope stage, and reciprocally. This co-localization technique offers a unique combination

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of acquiring the physical properties and chemical composition for samples at the same location

23

Lately, ED-HSI has been utilized in various studies for tracking


24-28

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Langmuir

with sub-micron spatial resolution.32, 35

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In this study, two types of MWCNT were selected (one with large and the other with small

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diameters, summarized in Table 1 to elicit different adsorption capacities for AAP. AAP was

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selected in this study as a representative type of PPCP compound because of its relatively high

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detection rates and levels in natural waters,36,

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CNT.38 The main objective of this study was to visualize and quantify adsorption of AAP

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molecules on CNT in aqueous suspensions using ED-HSI, AFM-Raman, and physicochemical

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models. Furthermore, this work is expect to provide novel insights into PPCP-CNT interactions

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and useful guidance for the design and application of CNT membrane filters for PPCP removal.

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The hypotheses are: (1) AAP molecules predominantly cover the exterior wall surfaces and

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co-localized at the regions of heterogeneous sites on CNT agglomerates, and (2) AAP appear to

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preferentially adsorb onto exterior sidewall of the individual L-MWCNT, rather than on the

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end-caps of L-MWCNT.

124

2. EXPERIMENTAL

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2.1 Materials

37

as well as modest adsorption capacity onto

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Reagent-grade acetaminophen (AAP) was purchased from Tokyo Chemical Industry CO.

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Two typical kinds of MWCNT (Beijing Boyu Technology Corporation of High-tech New

128

Materials, China) were used in this study. One has a thin wall but a large outer diameter

129

(L-MWCNT) and the other has a small outer diameter (S-MWCNT). The two MWCNTs were

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selected according to their typical diameters, which could explore the effect of MWCNT

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diameters on PPCP adsorption sites. According to the manufacturer, the L-MWCNT was 8 ACS Paragon Plus Environment

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synthesized with acetylene as the starting material and by crackle reaction with nickel catalyst.

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The preparation of S-MWCNT involved ultrasonic dispersion and centrifugal separation under

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the oxidation of H2SO4.

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2.2 Static adsorption experiment

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Static adsorption experiments were conducted following the approach of Liu et al.39 and Li

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et al.15 During each batch of experiments, 2.5 mg of L-MWCNT or S-MWCNT were added into

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a series of glass vials, each containing 30 mL of AAP solution with an initial AAP concentration

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of 0.25, 0.5, 1.0, 1.5, 3.0, 6.0 and 12.0 mg L-1. The vials were then sealed with caps lined with

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PTFE and mixed by end-over-end rotation in the dark, at an ambient temperature of 25  2 C

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for 99 h. Then, 3 mL of the above saturated adsorption solution was filtered with 0.22-μm PVDF

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membranes and the concentrations of AAP were measured to calculate the adsorption capacity of

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L-MWCNT and S-MWCNT. The AAP adsorption data for L-MWCNT and S-MWCNT used in

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this study were fit to the Langmuir model, Freundlich model and Sips model, respectively. The Langmuir adsorption equation is expressed as follows:40

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Qm kCe 1  kCe

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Qe 

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where Ce is the equilibrium AAP concentration in solution (mg L-1), Qe is the mass of AAP

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adsorbed per unit mass of MWCNT (mg g-1), K is a constant related to the energy of AAP

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adsorption to the MWCNT, and Qm is the maximum AAP adsorption capacity (mg g-1).

150

(1)

The Freundlich model assumes that the sorbent has a heterogeneous valance distribution 9 ACS Paragon Plus Environment

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Langmuir

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and then has different affinity for adsorption. Thia model takes the following form for a single

152

solute adsorption:41

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Qe  KCe

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where Qe is the mass of AAP adsorbed per unit mass of MWCNT (mg g-1), while Ce is the

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equilibrium AAP concentration in solution (mg L-1), K and 1/n are the Freundlich constants

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related to the sorption capacity and sorption intensity of the sorbent, respectively.

1/ n

(2)

The Sips model is another empirical model for the static adsorption and combines the

157 158

Langmuir- and Freundlich-type isotherm type models as below:41

159

Qe 

1/ n

Qm KCe 1/ n 1 KCe

(3)

160

where Ce is the equilibrium AAP concentration in solution (mg L-1), Qe is the mass of AAP

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adsorbed per unit mass of MWCNT (mg g-1), K is a constant related to the bonding energy of

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AAP to the MWCNT, and Qm is the maximum AAP adsorption capacity (mg g-1). The Sips

163

model can be extended to describe the multicomponent adsorption equilibrium data.

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2.3 Sample preparation

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Based upon the static adsorption results, L-MWCNT and S-MWCNT in the glass vials with

166

an initial AAP concentration of 6 mg L-1 were selected for ED-HSI characterization. The pristine

167

solution with an initial AAP concentration of 6 mg L-1, and then the AAP/CNT dispersions were

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further prepared using an ultrasonic processor (CPX 750, Cole-Parmer, USA) with a 1/2" (12.7

169

mm in diameter) probe for 40s on-26s off and totally 5 min at 40% power. The power output was 10 ACS Paragon Plus Environment

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28 W, yielding a power density of about 22 W cm-2 delivered by the probe. To better disperse the

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samples, the above CNT dispersions were also diluted by transferring 100 μL of original

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dispersion samples into 900 μL of deionized water. The diluted samples were then sonicated

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under the aforementioned condition for MWCNT re-dispersion.

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2.4 Dark-field hyperspectral imaging

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The prepared CNT dispersions were further analyzed using a BX-51 Olympus microscope

176

equipped with a hyperspectral imaging spectrophotometer (CytoViva Hyperspectral Imaging

177

System, Auburn, AL). We obtained spectral image files from 400 nm to 1,000 nm at 1.35-nm

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spectral resolution using the CytoViva® hyperspectral imaging module. CytoViva® hyperspectral

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image analysis software was utilized to quantify the spectral response of CNT and AAP, and

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further map them in the CNT/AAP mixtures.24 Firstly, the spectral endmembers or spectral

181

library for the components (CNT or AAP) was obtained from the sample image by choosing the

182

pixels that best represent the components; the hyperspectral image of each component in

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ultrapure water sample served as the guide for choosing the endmembers associated with the

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components of interest in the sample image. Finally, pixels mapped with spectral signatures from

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the same library were identified as of the same material with a threshold of 0.85. Pixels of the

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same material were colored with the same pseudocolor on an enhanced dark-field image, i.e.

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L-MWCNT or S-MWCNT were colored red and AAP were colored purple. The percentage of

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pixels mapped as L-MWCNT, S-MWCNT, AAP or unidentified were calculated with ENVI

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software (version 3.2, Research System Inc.). The details of the steps involved in hyperspectral


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Langmuir

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image analysis can refer to a previous study by Badireddy et al.20

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2.5 Other characterization and analytical methods

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Transmission electron microscope and scanning electron microscope. For the TEM

193

measurement of MWCNT, 0.25 mg MWCNT was added into 30 mL ethanol and sonicated for

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30 min with a water-bath sonicator (KQ5200DE, Kunshan shumei, China) at a frequency of 10

195

kHz to obtain a uniform MWCNT dispersion. Samples of MWCNT were prepared by depositing

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the above dispersion on a copper substrate. The morphology and structure of MWCNT were then

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characterized with the TEM (FEI TF 20, Thermo Fisher, USA). Besides, the air-dried MWCNT

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samples were sputter-coated with gold and then imaged using a cold cathode field emission

199

scanning electron microscope (SEM, S-4800, HITACHI, Japan) at 10 keV.

200 201 202

X-ray photoelectron spectroscopy. Surface chemical composition and functional groups of CNT were determined by an X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, USA). Thermogravimetric

analysis

and

Brunauer-Emmett-Teller

measurement.

For

the

203

determination of CNT purities by the thermogravimetric analyzer (HCT3, Beijing Heven

204

Scientific Instrument Factory, China), 10 mg MWCNT were placed in an aluminum oxide pan

205

and heated at a heating rate of 10 C min-1 to 1100 C in an atmosphere with air flowing at 180

206

mL min-1. For Brunauer-Emmett-Teller (BET, AutosorbiQ, Quantachrome Instruments, USA)

207

measurements of the specific surface area (SSA), N2 adsorption data were obtained at 77 K using

208

a high-resolution gas adsorption analyzer with high vacuum capacity (5×10-7 Pa), following the

209

standard method for black carbon samples.42

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Acetaminophen concentration analysis. The UV absorbance of the feed solution and the 12 ACS Paragon Plus Environment

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liquid supernatant after adsorption were measured using a UV-vis spectrophotometer (DR 6000,

212

HACH, USA) at a wavelength of 242 nm to determine AAP’s aqueous concentrations.

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Atomic force microscopy-Raman measurements. Integration of AFM with Raman can

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simultaneously generate topographical and chemical mapping of the same sample area at

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nanoscale. With a co-localized AFM-Raman system (NTEGRA Spectra, NTMDT, Russia), the

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chemical distribution of AAP and S-MWCNT was expected to be resolved with a higher

217

resolution and sensitivity. The detailed procedure was described in SI.

218

2.6 Statistical analysis

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For each sample, ED-HSI spectra and images were obtained at three or more different

220

locations. The average surface coverage of AAP and MWCNT in ED-HSI images was calculated

221

and presented as average value ± standard deviation for discussions. t-test was conducted to

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determine the effects of dilution on the surface coverage and the differences were considered

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significant when p < 0.05.

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3. RESULTS AND DISCUSSIONS

225

3.1 Characteristic of the multi-walled carbon nanotubes

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The respective morphologies of L-MWCNT and S-MWCNT were determined by SEM and

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TEM. Figs. 1a & 1b show that the L-MWCNT exhibit curved and filamentous structure, while

228

the S-MWCNT are primarily granular aggregates. TEM images (Fig. 1c & 1d) further reveal the

229

thin wall but large outer diameters of L-MWCNT and the smaller outer diameters of S-MWCNT.

230

L-MWCNT and S-MWCNT are shown to possess outer diameters of 40-210 nm and 5-15 nm,


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Langmuir

respectively.

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Table 1 summarizes some major properties of the two MWCNT samples. According to the

233

XPS analysis, S-MWCNT possesses higher surface oxygen content than L-MWCNT (3.37% vs

234

2.38%), while with similar surface nitrogen content (0.48% vs 0.45%). Furthermore, S-MWCNT

235

has a larger specific surface area (SSA) than L-MWCNT due to the smaller outer diameter of

236

S-MWCNT than L-MWCNT, and larger pore volume of S-MWCNT than L-MWCNT.

237


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a. L-MWCNT

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b. S-MWCNT

000

c. L-MWCNT

d. S-MWCNT

200 nm

20 nm

f. S-MWCNT

e. L-MWCNT

Fig. 1 SEM images of (a) L-MWCNT and (b) S-MWCNT; TEM images of (c) L-MWCNT and (d) S-MWCNT; XPS C1s spectra of (e) L-MWCNT and (f) S-MWCNT. Deconvolution of the spectra suggests the existence of five types of carbon species bonds, including: 1. C=C (graphite), 2. C-C (sp3), 3. C-OH (hydroxyl groups), 4. C=O (carbonyl groups), and 5. COO


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Langmuir

(carboxyl groups). 238 239

Table 1. Characteristics of carbon nanotubes used in the study.

244

Outer Lengthb Surface Surface BET-Sd diametera (μm) oxygenc nitrogenc (m2/g) (nm) (%) (%) L-MWCNT 40-210 1-10 2.38 0.45 153 S-MWCNT 5-15 10-30 3.37 0.48 360 a Data based on the TEM images and analyzed by the ImageJ software. b Data from the manufacturer. c Data determined by XPS. d Data obtained from BET measurements. e Data obtained by a thermogravimetric analyzer.

245

3.2 Adsorption capacities of MWCNT

CNT type

240 241 242 243

Pore volumed (cm3/g) 0.027 0.056

Puritye (%) > 90 > 95

246

Because the correlation coefficients (r2) of L-MWCNT and S-MWCNT of Sips model and

247

Freundlich model were higher than that of Langmuir model, the Langmuir model was less

248

accurate than Freundlich isotherm model and Sips model in fitting the experimental adsorption

249

data (Fig. 2 & Table S2a); this indicates that AAP adsorption onto MWCNT was not

250

homogeneous. Overall, the Sips model had the best agreement with the experimental data, which

251

yielded maximum AAP adsorption capacities of 54.6 mg g-1 and 252.7 mg g-1 for L-MWCNT

252

and S-MWCNT, respectively (Table S2b). Meanwhile, at an initial AAP concentration of 6 mg

253

L-1, the AAP adsorption capacities of L-MWCNT and S-MWCNT were 21.6 mg g-1 and 57.8 mg

254

g-1, respectively. Accordingly, the removal efficiencies of L-MWCNT and S-MWCNT for AAP

255

were 46% and 80%, respectively. The higher removal efficiency of S-MWCNT than that of

256

L-MWCNT was attributed to the greater SSA of S-MWCNT (Table 1). This finding was


Langmuir

257

consistent with the work by Wei et al., who found that the adsorption capacity of MWCNT for

258

diclofenac sodium and carbamazepine increased with the SSA.43

259 100

a. Langmuir

Qe (mg g-1)

80

Model

LangmuirEXT

Equation

y = 1/(a + b*x^ (c-1)) 11.50604

Reduced Chi-Sqr

L-MWCNT (r2=0.891) S-MWCNT (r2=0.988)

60

0.89099

Adj. R-Square a b

B

c a b

D

40

c

20 Original AAP Conc.=6 mg L-1

0

0

1

2

3

4

5

-1

Ce (mg L )

100

b. Freundlich Model

80

Equation

2

Qe (mg g-1)

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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L-MWCNT (r =0.982) S-MWCNT (r2=0.989)

60

Reduced Chi-Sqr Adj. R-Squ

B

40

D

20 Original AAP Conc.=6 mg L-1

0

0

1

2

3

4

-1

Ce (mg L )


5

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100

c. Sips

80

Qe (mg g-1)

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

L-MWCNT (r2=0.985) S-MWCNT (r2=0.989)

60 40 20

Original AAP Conc.=6 mg L-1

0

0

1

2

3

4

5

-1

Ce (mg L )

Fig. 2 Variations in the adsorption capacity of L-MWCNT and S-MWCNT with equilibrium solution concentrations of AAP. The curves represent least-square fitting of duplicate experimental data into the (a) Langmuir model, (b) Freundlich model and (c) Sips model, as well as their corresponding parameters are summarized in Table S2a and Table S2b. Temperature = 25 ± 2 °C, initial pH = 7.0. 260

3.3 Effect of AAP adsorption on CNT morphology

261

AAP adsorption on the MWCNT exerted a noticeable effect on CNT morphology. Fig. 3a

262

exhibited that, the L-MWCNT dispersion in deionized water was composed of some dispersed

263

CNT and small CNT aggregates (7-11 μm). After the adsorption of AAP, the aggregates became

264

considerably larger (23-34 μm) (Fig. 3b). Similarly, the adsorption of AAP onto S-MWCNT led

265

to large CNT aggregates (7-39 μm) with floc-like structures (Fig. 3d). In a previous study,

266

Oleszczuk et al. also found that the CNT adsorption of PPCP compounds (e.g., oxytetracycline

267

and carbamazepine) caused reorganization of the CNT aggregates and changed their

268

morphology.43 Besides, aqueous organic matter has been well-known as an effective dispersing 18 ACS Paragon Plus Environment

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agent for CNT.45 For example, bovine serum albumin (BSA) and humic acid (HA) were reported

270

to reduce the MWCNT aggregations through the strong steric hindrance.46 Compared to AAP,

271

BSA and HA had relatively inflexible structures and high charge densities, which increased

272

steric hindrance and/or electrostatic repulsion between MWCNT after their sorption. Given its

273

small molecular size and high pKa value (~10.3), AAP mainly existed as molecular at pH 7,47

274

thus reducing the electrostatic repulsion between the MWCNT adsorbed by AAP and facilitating

275

the MWCNT aggregations after the adsorption of AAP. a. L-MWCNT

b. L-MWCNT /AAP

10 μm

10 μm

d. S-MWCNT/AAP

c. S-MWCNT

10 μm

10 μm 276 277

Fig. 3 Comparison of the enhanced darkfield hyperspectral images of (a) L-MWCNT and (b) 19 ACS Paragon Plus Environment

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L-MWCNT/AAP; (c) S-MWCNT and (d) S-MWCNT/AAP dispersions without classification.

279

3.4 Observing AAP adsorption onto CNT agglomerates

280

Firstly, the spectral endmembers or spectral library for AAP, L-MWCNT and S-MWCNT

281

components were obtained from pure AAP, L-MWCNT and S-MWCNT dispersions in

282

deionized water, respectively (Fig. S1, Supporting Information). The spectral library was served

283

as the references to match the spectra of components in the AAP/CNT mixture after the

284

adsorption equilibrium. The endmembers that showed a match of greater than 75% with spectral

285

library of the components were considered to represent the components in the AAP/CNT

286

mixtures. In the hyperspectral images, CNT were pseudocoloured red while AAP were colored

287

purple to illustrate the spatial localization of CNT and AAP.20

288

Fig. 4 and Fig. 5 show the typical hyperspectral maps of L-MWCNT/AAP and

289

S-MWCNT/AAP in adsorption equilibrium under aqueous conditions, respectively. Fig. 4a

290

indicates that most of AAP (purple) lay around L-MWCNT (red). The area for AAP accounts for

291

77.2 ± 4.8% of the total colored area for both L-MWCNT and AAP, while the surface coverage

292

of L-MWCNT is 22.8 ± 4.8%. Then the surface coverage ratio of AAP/L-MWCNT was

293

calculated to be ~3.4. The further zoom-in images (Fig. 4b-c) reveal two main scenarios: (1)

294

AAP is laying around CNT, and (2) AAP is surrounded by CNT. These results indicate that,

295

even in an equilibrium adsorption condition, the adsorbed AAP molecules are heterogeneously

296

distributed on the external surfaces of L-MWCNT agglomerates, rather than to form a

297

homogeneous layer on CNT surfaces, as it is predicted by the Sips model (Fig. 2).


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298

Fig. 2 exhibits that the S-MWCNT have a higher adsorption capacity to AAP than the

299

L-MWCNT. Correspondingly, the ED-HSI image shows that S-MWCNT accounts for a greater

300

portion of the total colored area against the dark field compared to L-MWCNT (Fig. 5a).

301

Specifically, AAP covers 52.3 ± 4.4% of the total classified area while the S-MWCNT occupies

302

47.8 ± 4.4%. Thus the surface coverage ratio of AAP/S-MWCNT was calculated to be ~1.1,

303

which indicated that there were more available adsorption sites of S-MWCNT than L-MWCNT

304

when the adsorption reached equilibrium with the aqueous phase. Overall, these ED-HSI results

305

demonstrate the heterogeneous nature of PPCP adsorption, which has commonly been

306

overlooked in related studies.17-19


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Langmuir

a

1

CNT (Red): 25.7% AAP (Purple): 74.3%

b

2

3 10 μm

Red: 22.8% Purple: 77.2%

1.25 μm

d

c

Red: 27.5% Purple: 72.5%

1.25 μm

Red: 18.0% Purple: 82.0%

1.25 μm

307 308

Fig. 4 Hyperspectral imaging (a) with spectrally identified L-MWCNT as red pixels and AAP as

309

purple pixels. (b-d) is the magnified images of location 1-3 in the full image of (a), respectively.

310

The purple represents AAP and the red represents S-MWCNT.


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a

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b


10 μm

Red: 46.3% Purple: 53.7%

1.25 μm

d

c

Red: 52.7% Purple: 47.3%

Red: 44.37% Purple: 55.63% 1.25 μm

1.25 μm 311 312

Fig. 5 Hyperspectral imaging (a) with spectrally identified S-MWCNT as red pixels and AAP as

313

purple pixels. (b-d) is the zoom-in images of location 1-3 in the full image of (a), respectively.

314

3.5 Observing AAP adsorption onto individual CNT

315

To directly observe the adsorption sites on individual CNT, the CNT/AAP samples in

316

adsorption equilibrium were diluted by ten times with deionized water to mitigate the

317

aggregation of L-MWCNT. Compared with AAP adsorption observed on CNT aggregates 23 ACS Paragon Plus Environment

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Langmuir

318

without dilution, the surface coverage of L-MWCNT slightly increased from 22.8 ± 4.8% in the

319

original sample to 27.0 ± 11.2% in the diluted sample (Fig. 4 vs Fig. 6). Similarly, the

320

S-MWCNT percentage increased from 47.8 ± 4.4% in the original sample to 59.4 ± 3.8% in

321

diluted sample (Fig. 5 vs Fig. 7). The increased percentage of CNT pixels and the corresponding

322

decreased percentage of AAP pixels suggested that more adsorption sites on MWCNT were

323

exposed and the adsorption capacity of MWCNT for AAP decreased as the CNT dispersion was

324

diluted, which was consistent with the variations in adsorption capacity obtained in the static

325

adsorption experiment (Fig. 2). In summary, the increased CNT percentages in the dilution

326

further verified the heterogeneity of AAP adsorption onto CNT.

327

Due to the limited resolution of ED-HSI and the small diameter of S-MWCNT, we did not

328

find individual S-MWCNT tubes or observe the adsorption sites of S-MWCNT in Fig. 7. To

329

further reveal the AAP adsorption sites on S-MWCNT, the topography and Raman mapping

330

images of CNT and AAP were obtained with a co-localized AFM-Raman system. Fig. S3a

331

shows the morphology of the sample substrate (PVDF membranes), where the brightened dots in

332

Fig. S3b and Fig. S3c are likely MWCNT, which yields strong signals of the D and G bands in

333

Fig. S3e that do not exist in the clean PVDF membrane (data not shown). The large dimension of

334

these brightened dots suggests that S-MWCNT was presented as relatively large aggregates

335

when being filtered on the substrate. Moreover, the brightened dots as marked with yellow

336

arrows in Fig. S3d could be AAP, which elicits a weak Raman peak at 650 cm-1, an assignment

337

to the phenyl ring bend in AAP (Fig. S3f).51 Though we verified the presence of AAP on

338

S-MWCNT, the adsorption sites of S-MWCNT could not be clearly resolved, either. Therefore, 24 ACS Paragon Plus Environment

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339

AFM-Raman did not demonstrate a better resolution than ED-HSI but it still could be utilized as

340

an assistant tool for detection and characterization of PPCP/CNT adsorption system.

a

b

3 2

1 CNT (Red): 30.5% AAP (Purple): 69.5%

10 μm

341

1.25 μm

d

c

Red: 35.3% Purple: 64.7%

Red: 31.3% Purple: 68.7%

1.25 μm

Red: 14.2% Purple: 85.8%

1.25 μm

342

Fig. 6 Hyperspectral imaging with spectrally identified L-MWCNT (a) as red pixels and AAP as

343

purple pixels for the dilution mixture samples after 10X dilution. (b-d) is the zoom-in images of

344

location 1-3 in the full image (a), respectively.


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Langmuir

a

2

3

b

1


10 μm

Red: 62.2% Purple: 37.8%

1.25 μm

d

c

Red: 55.1% Purple: 44.9%

Red: 61.0% Purple: 39.0% 1.25 μm

1.25 μm

345 346

Fig. 7 Hyperspectral imaging with spectrally identified S-MWCNT (a) as red pixels and AAP as

347

purple pixels for the dilution mixture samples after 10X dilution. (b-d) is the magnified images

348

of location 1-3 in the full image (a), respectively.

349

To verify that the rod-shape objects in Fig. 6 were individual L-MWCNTs, silver (Ag)

350

nanoparticle was used as the reference to clarify the rod size (Fig. S2). Based on the

351

magnification ratio of Ag nanoparticles (Fig. S2a), the diameter of L-MWCNT was calculated to

352

be 125 nm, which was among the range of outer diameter results (40-210 nm) obtained by TEM. 26 ACS Paragon Plus Environment

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353

Besides, the spectra of various interested locations in Fig. 6d were extracted to verify the

354

distribution of CNT and AAP. Fig. 8 shows the spectra extracted on the spots pointed with the

355

yellow arrows corresponded well to the typical spectral curves of pure L-MWCNT (Fig. S1).

356

Meanwhile, the spectra at locations with the white arrows were consistent with the characteristic

357

spectral curves of pure AAP in Fig. S1. These results indicated that AAP indeed adsorbed onto

358

the CNT, and most of AAP were found to surround the CNT aggregates, which may be attributed

359

to the easier adsorption sites of the CNT aggregates edges than the CNT aggregates central.

360

These results further verified the CNT adsorption model proposed by Pearce et al. and Rols et

361

al., who found that, for the close-capped CNT samples, the adsorption of Helium and Argon

362

firstly took place on the interstitial channels and the external surfaces, and then on the internal

363

sites of the CNT bundles.48, 49


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a

b

1.25 μm

1.25 μm

1600 1400

1600

c. CNT

1200

1200

1000

1000

800 600 400

364

800 600 400 200

200 450

d. AAP

1400

Intensity (nm)

Intensity (nm)

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

500

550

600

650

700

0 450

750

500

550

Wavelength (nm)

600

650

700

750

Wavelength (nm)

365

Fig. 8 Hyperspectral imaging of the AAP/L-MWCNT dilution mixture samples after 10X

366

dilution, (a) the yellow arrow pointed to the L-MWCNT (colored in red), (b) the white arrow

367

pointed to AAP (colored in purple), (c) the corresponding spectral means for the locations

368

pointed by the yellow arrows in Fig. a, and (d) the corresponding spectral means for the locations

369

pointed by the white arrows in Fig. b.

370

The above-mentioned results of the AAP adsorption on individual CNT (Fig. 8) implied that

371

AAP primarily adsorb on the sidewall other than on the caps of the individual L-MWCNT,

372

probably due to the stronger π-π interaction between AAP and the CNT sidewalls. Zhao and Lu

373

also found that the π-π electron coupling between adsorbate molecules and CNT sidewall 28 ACS Paragon Plus Environment

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374

surfaces dominated the adsorption process, through experimental study and density functional

375

theory calculation.50 Pan and Xing also declared that π-π bond was the dominating interaction

376

force for the adsorption of chemicals containing benzene rings on CNT and different numbers of

377

π electrons could reflect the contribution of π-π interactions, and the chemical compounds

378

adsorption increased as the number of π electron donor-acceptor increased.51 Since the caps of

379

CNT usually possess smaller π acceptor or π donator than the sidewalls due to the smaller

380

contact area of CNT area, the π-π interactions between AAP and CNT caps should also be

381

weaker than those between AAP and CNT sidewalls. Overall, the different AAP adsorption on

382

CNT surface and CNT caps further indicated the heterogeneous nature of AAP adsorption on

383

CNT.

384

4. CONCLUSION

385

For the first time, we investigated the AAP adsorption behavior and adsorption sites onto

386

CNT using a novel characterization method, dark-field ED-HSI. The results demonstrated the

387

heterogeneity of AAP adsorption onto CNT aggregates and individual large-diameter carbon

388

nanotubes. The heterogeneity of adsorption was observed from two perspectives: (1) most of

389

CNT existed as aggregates during AAP adsorption; and (2) individual L-MWCNT presented in

390

the diluted dispersion, and AAP appeared to primarily adsorb onto the sidewall, rather than the

391

caps of L-MWCNT. Heterogeneous adsorption usually occurred at the solid-gas interface and

392

could explain the adsorption mechanism intrinsically, which was mainly developed and

393

well-established based on mathematical methods in the past decade. These nanoscale

394

observations in our study provided important insights into the adsorption behaviors of 29 ACS Paragon Plus Environment

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Langmuir

395

carbonaceous nanomaterials for AAP and other organic contaminants of emerging concerns. In

396

addition, the new characterization method of dark-field ED-HSI is demonstrated to be effective

397

ways for the nanoscale adsorption mechanism study and co-localized AFM-Raman could be

398

applied as a supporting tool. For example, the new characterization methods could be used to

399

compare the contaminants sorption and desorption, biomolecular binding on carbonaceous

400

nanomaterials.

401 402

ACKNOWLEDGEMENT

403

This work was supported by the Fundamental Research Funds for the Central Universities

404

(Grant No. 310421111), National Natural Science Foundation of China (Grant No. 51778055)

405

and the US National Science Foundation (Grant No. 1756444).

406 407 408 409

ASSOCIATED CONTENT

410

Supporting Information

411

Fig. S1 is the comparison of the spectral means of the pristine AAP, L-MWCNT and

412

S-MWCNT. Fig. S2 represents the (a) Silver (Ag) nanoparticle served as the reference in the in

413

low resolution (×100) and their corresponding magnified image in high resolution (×2,500), with

414

the magnification ratio of 10:1; (b) the individual L-MWCNT fiber with a calculated diameter of

415

125 nm based on the Ag magnification ratio. Fig. S3 is the Co-localized AFM-Raman 30 ACS Paragon Plus Environment

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416

measurements of (a) AFM topography image; (b) Raman map of the D band of MWCNT

417

collected in the range 1300-1370 cm-1; (c) Raman map of the G band of MWCNT collected in

418

the range 1560-1610 cm-1; (d) Raman map collected in the range 635-660 cm-1 which

419

corresponds to the peak of Acetaminophen at 650 cm-1; and Raman spectrum of S-MWCNT (e)

420

before and (f) after the adsorption of AAP. Table S1 summarizes the relative percentage of

421

carbon atoms contributed to the five C1s peaks determined by XPS analyses of L-MWCNT and

422

S-MWCNT. Table S2a is the comparison of the coefficients of correlation (r2) associated with

423

least-square fitting of the experimental results into the Langmuir model, the Freundlich model

424

and the Sips model for the adsorption of AAP by L-MWCNT and S-MWCNT. Table S2b is the

425

comparison of the constants related to the bonding energy of AAP to the MWCNT (K),

426

equilibrium surface concentrations (Qe), and coefficients of correlation (r2) associated with the

427

Sips model for the adsorption of AAP by L-MWCNT and S-MWCNT.

428

429

ORCID number:

430

Yifei Wang: 0000-0002-3386-7677; Wanyi Fu: 0000-0001-9653-4012; Wen Zhang: 0000-0001-8413-0598; Appala Raju Badireddy: 0000-0003-4174-2767; Haiou Huang: 0000-0001-9943-0825.

431 432 433 434

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Abstract Graphic

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Hyperspectral imaging of acetaminophen on multi-walled carbon

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