Methoxy Polyethylene

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Letter

Nanocomposites of Molybdenum Disulfide/Methoxy Polyethylene Glycol-co-Polypyrrole for Amplified Photoacoustic Signal Hohyeon Lee, Haemin Kim, Thang Phan Nguyen, Jin Ho Chang, Soo Young Kim, Hyuncheol Kim, and Eunah Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10763 • Publication Date (Web): 18 Oct 2016 Downloaded from http://pubs.acs.org on October 19, 2016

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ACS Applied Materials & Interfaces

Nanocomposites of Molybdenum Disulfide/Methoxy Polyethylene Glycol-co-Polypyrrole for Amplified Photoacoustic Signal Hohyeon Leea+, Haemin Kimb+, Thang Phan Nguyend, Jin Ho Changb,c, Soo Young Kimd*, Hyuncheol Kima,b*, Eunah Kang*d a

Department of Chemical & Biomolecular Engineering

b

c

Department of Biomedical Engineering

Sogang Institute of Advanced Technology, Sogang University, 35 Baekbeom-ro, Mapo-gu

d

School of Chemical Engineering and Material Science, Chung-Ang University, 221 Heukseok-

Dong, Dongjak-Gu, Seoul, Korea

Eunah Kang School of Chemical Engineering and Material Science, Chung-Ang University, 221 HeukseokDong, Dongjak-Gu, Seoul, Korea [email protected] Hyuncheol Kim Department of Chemical & Biomolecular Engineering, Sogang University, 35 Baekbeom-ro, Mapo-gu, Seoul 121-742, South Korea Biomedical Engineering, Sogang University, 35 Baekbeom-ro, Mapo-gu, Seoul 121-742, South Korea [email protected] Soo Young Kim School of Chemical Engineering and Material Science, Chung-Ang University, 221 HeukseokDong, Dongjak-Gu, Seoul, Korea [email protected]

+

equally contributed to these author

ACS Paragon Plus Environment


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Abstract Photoacoustic activity is the generation of an ultrasonic signal via thermal expansion or bubble formation, stimulated by laser irradiation. Photoacoustic nanoplatforms have recently gained focus for application in bio-electric interfaces. Various photoacoustic material types have been evaluated, including gold nanoparticles, semiconductive π-conjugating polymers (SP), etc. In this study, surfactant-free methoxy-polyethylene glycol-co-polypyrrole copolymer (mPEG-coPPyr) nanoparticles (NPs) and mPEG-co-PPyr NP/molybdenum disulfide (mPEG-co-PPyr/MoS2) nanocomposites (NCs) were prepared and their photoacoustic activity was demonstrated. The mPEG-co-PPyr NPs and mPEG-co-PPyr/MoS2 NCs both showed photoacoustic signal activity. The mPEG-co-PPyr/MoS2 NCs presented a higher photoacoustic signal amplitude at 700 nm than the mPEG-co-PPyr NPs. The enhanced photoacoustic activity of the mPEG-co-PPyr/MoS2 NCs might be attributed to heterogeneous interfacial contact between mPEG-co-PPyr and the MoS2 nanosheets due to complex formation. Laser ablation of MoS2 might elevate the local temperature and facilitate the thermal conductive transfer in the mPEG-co-PPyr/MoS2 NCs, amplifying PA signal. Our study, for the first time, demonstrates enhanced PA activity in SP/transition metal disulfide (TMD) composites as photoacoustic nanoplatforms. KEYWORDS: nanocomposites, photoacoustics, polypyrrole, MoS2, transition metal disulfide, semiconductive π-conjugating polymers


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Semiconductive polymers have mainly been exploited for electric materials and devices. Recently, semiconductive π-conjugating polymers (SPs) have gained focus due to their energy absorbing and emitting activity that can potentially be applied in the fields of medicine and biology.1 SPs were initially fabricated as water-dispersible nanodots or nanoparticles (NPs) with functions such as near infrared (NIR) fluorescence, luminescence, and photoacoustic properties, etc.1-2

Though SPNPs are not biodegradable and their real in vivo application remains a distant

prospect, well-designed optical SPNPs still offer advantages for specific objectives with minimized toxicity to bioorganisms. Local diagnostic applications beyond the optical cutoff boundary might be expedited by SPNPs. Photoacoustic (PA) imaging has emerged as an alternative to optical imaging. Optic-based NIR fluorescence imaging is limited in terms of the tissue-penetrating depth and spatial resolution, whereas the PA approach is based on laser-induced excitation and receiving an ultrasound signal, allowing achievement of greater imaging depth with better tissue penetration and higher spatial resolution. Though endogeneous agents (hemoglobin, melanin) are active for PA imaging, multiple functions of targeting, organ specificity, or molecule-specific imaging require exogeneous PA-active platforms. Recently, photoacoustic theranostics have emerged as a multifunctional imaging modality and drug releasing reservoir3, and based on new synthetic tools, SPNPs have earned distinction as a prospective candidate for exploitation in porphyrinconjugated lipid vesicles4, hybrids with carbon-based materials5-6 or inorganic/metal-based nanoparticles, 4, 6-8 and small chemical dyes (ICG).9 Specifically, semiconductive polypyrrole has been implemented as an electroactive platform at the interface between materials and biological events.10

Further, polypyrrole is known to



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absorb energy and transfer it into the environment as a thermal or photoacoustic signal.11-14 Tuning of the optical properties may also be achieved depending on the electron donor-acceptor backbone.11, 15

Though solid, hard-type, water-insoluble SPNPs must be completely cleared

from living organisms, SPNPs with water stability can prospectively be applied in photoacoustic flow cytometry or photoacoustic imaging agents. On the other hand, two-dimensional inorganic materials16 or transition-metal dichalcogenides including MoS2, WS2, and TiS2 etc., have gained prominence in materials science, exhibiting potential in nanomedicine, biosensors,17 and biomedical imaging.18-20 Two-dimensional transition metal disulfides (TMDs) have a sandwich structure comprising chalcogen atoms with an hexagonal layer of metal atoms. Exfoliated transition metal nanosheets have shown enhanced electrochemical current density and thus expedite charge transfer.21 Because of their characteristic electrochemical and optical properties, TMDs are known to absorb near infrared energy and can be employed for photothermic therapy and photoacoustic signaling in combination with integrated chemical conjugation with biomaterials.18-20, 22-23 However, integration of SP and two-dimensional TMDs has not been exploited for harnessing the synergistic effect of photoacoustic and photothermal activity in prior studies. This study presents the synthesis of a semi-soluble polypyrrole-co-PEG copolymer (mPEG-co-PPyr) that can be formulated into nanoparticles with inorganic materials in the absence of an exogenous surfactant. Two-dimensional molybdenum disulfide nanosheets are incorporated into mPEG-coPPyr NPs by solvent exchange during nanoprecipitation. The photoacoustic signals of PPyr-PEG NP and MoS2-incorporated mPEG-co-PPyr NCs (mPEG-co-PPyr/MoS2) are investigated to suggest a new type of photoacoustic signaling agent with synergistic potential.


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The semi-soluble mPEG-co-PPyr copolymer was synthesized for formation of surfactant-free nanoparticles. The hydroxyl end group of mPEG-OH was subjected to the initiation reaction with butyl lithium and reacted with 2-(trichloroacetyl)pyrrole to prepare pyrrole-terminated mPEG (Scheme 1). Nuclear magnetic resonance (NMR) analysis shows peaks (δ = 6.1, 7.2, 7.6, 9.5 ppm) of the pyrrole moiety, providing evidence of pyrrole termination on mPEG (Figure 1). Pyrroleterminated mPEG was copolymerized with pyrrole in the presence of ferric paratoluenesulfonate [Fe(PTS)3] in methylene chloride (MC). The similar oxidation potentials of pyrrole and pyrroleterminated mPEG might contribute to copolymerization, evidenced by the color change to black. The final product (mPEG-co-PPyr) was homogeneously dispersed in MC, which is semi-soluble in organic solvents. Exfoliated MoS2 nanosheets were prepared from MoS2 powder by ultrasonication in N-vinylpyrrolidone (NVP). After removal of NVP and unexfoliated MoS2, dried MOS2 nanosheets were re dispersed in methylene chloride to formulate with mPEG-coPPyr. Synthesized mPEG-co-PPyr

was confirmed from Raman peak at 1588 cm-1 (C=C), 1421

cm-1 and 1329 cm-1 (antisymetrical C-N bond), 993 cm-1(ring formation), and 1052 cm-1 (C-H bending). Raman resonance of MoS2 nanosheets was also observed at 381 and 407 cm-1 (Figure S1.) The water-dispersible complexes mPEG-co-PPyr and mPEG-co-PPyr/MoS2 were prepared by nanoprecipitation through solvent exchange. The morphology of the MoS2 nanosheets, mPEGco-PPyr NPs, and mPEG-co-PPyr/MoS2 NCs was characterized using TEM. The TEM images show the two-dimensional planar features of the MoS2 nanosheets that comprised several layers of nanosheets with minimal folding of the layers. The size of the MoS2 nanosheets ranged from 300 to 500 nm based on the TEM images (Fig. 2a). The MoS2 nanosheets had a crystalline structure with several molecular layers stacked in a planar form. Semi-soluble mPEG-co-PPyr



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and its mixture with MoS2 nanosheets in methylene chloride was prepared at a desired concentration and precipitated in water under ultrasonication. Solvent exchange based on solubility differences induced generation of the nanoparticulate form of mPEG-co-PPyr and mPEG-co-PPyr/MoS2. The mPEG-co-PPyr NP had a spherical morphology. The particle surface appeared relatively smooth compared to that previously reported,24 probably due to the PEGylation. Interestingly, the mPEG-co-PPyr/MoS2 nanocomplexes had a sheet-like, edge-onthe-sphere-like particulate structure (Fig. 2a). The morphology of mPEG-co-PPyr/MoS2 indicates that mPEG-co-PPyr was physically adsorbed on the MoS2 nanosheets in the mixture solution and the MoS2 nanosheets might be folded into integrated particles with mPEG-co-PPyr during particle formation under mechanical stimulation and solvent exchange. The average hydrodynamic diameter of mPEG-co-PPyr and mPEG-co-PPyr/MoS2 measured by dynamic light scattering (DLS) was 475±13.8 nm (PDI 0.47) and 447 ±17.2 nm (PDI 0.43), respectively (Fig. 2b). The addition of MoS2 nanosheets did not significantly increase the size of the complexes. The zeta potential of the mPEG-co-PPyr NPs was only 7.5 mV, which is close to neutral, indicating that the PEG layer of the mPEG-co-PPyr NPs projected into the outer water interface, masking the positive charges of polypyrrole. In contrast, the mPEG-co-PPyr/MoS2 nanocomplexes had a higher zeta potential of 25.2 mV. The incorporated MoS2 nanosheets were not completely enveloped by mPEG-co-PPyr, leading to incompletely sphere-like NPs. Exposed planar MoS2 on the outer surface of the mPEG-co-PPyr/MoS2 NCs accounts for the strongly positive zeta potentials. The UV-vis spectrophotometric properties of mPEG-co-PPyr and mPEG-co-PPyr/MoS2 were studied (Fig. 2c). Both spectra show an absorption band in the range of 300 to 350 nm,


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corresponding to the π*-π transition, indicating the presence and formation of polypyrrole. A broad absorption band spanning 450 nm to 750 nm was seen in the spectrum of mPEG-co-PPyr, derived from bipolaron transition from the bication, as an oxidized state of polypyrrole.25 The UV-vis spectrum of the water dispersion of the mPEG-co-PPyr/MoS2 NCs showed shoulders at 413 nm and 497 nm. It is reported that the typical UV spectrum of MoS2 nanosheets in organic solvent shows two peaks at approximately at 610 and 671 nm, induced by energy splitting of the valence band and spin orbital coupling.21

MoS2 bulk powder also shows two peaks at 693

and 645 nm. Interestingly, the two separated peaks of the mPEG-co-PPyr/MoS2 NCs showed clear blue shifts to the region of 400 nm. This result is notable in that the mPEG-co-PPyr/MoS2 NCs still possess properties of the transition metal disulfide, i.e., MoS2 nanosheets, showing two separated peaks. The peak shifts might be due to the interaction with PPyr, interfacing with the electron and bication. Partially intact MoS2 probably exists as few layers of sheets within the mPEG-co-PPyr/MoS2 NCs, generating heterogeneous nanosurface contact with polypyrrole. Formation of the mPEG-co-PPyr/MoS2 NC complex induced the blue shift in the optical spectra. Overall, the UV absorbance of the mPEG-co-PPyr/MoS2 NCs was high in the measured range, compared to that of the mPEG-co-PPyr NPs at the same concentration. The photoacoustic signal activity of the mPEG-co-PPyr NPs and mPEG-co-PPyr/MoS2 NCs was investigated by using a pulse laser (Scheme 2). The average energy density of the input laser power was 14.48 mJ·cm-2 at the wavelength of 700 nm. The photoacoustic signal amplitude of mPEG-co-PPyr was investigated with varied concentrations from 50 to 600 µg·ml-1. Because the PA signals at high (70%) and low energy (30%) were measured using the same mass concentration, the PA signal was higher with high energy irradiation than with low energy irradiation (Fig. 3). Specifically, the amplitude of the PA signal of mPEG-co-PPyr (100 µg·ml-1



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in water) was 112 at high energy and 41 at low energy. The limit of detection was 25 µg·ml-1 for the mPEG-co-PPyr NPs in water, which was similar to the background signal with water. The PA signal amplitude was linearly enhanced with increasing mass concentration, indicating that the mPEG-co-PPyr NPs were photoacoustically activated. Linear regression analysis of the amplitude of the PA signal of mPEG-co-PPyr with variation of the mass concentration suggested potential quantitative applicability as an imaging agent, as polypyrrole possesses photoacoustic properties. Generally, thermal aging of polypyrrole induces reduction of the conductivity, proposed to originate from reduction of the grain size, where the grains can act as tunnels for charge transport across the insulating gap.26-27 Reduced conductivity has been observed due to oxygen adsorption on the surface of polypyrrole fibrils and diffusion into the interior. This mechanism might proceed by a similar pathway to that of laser-induced high thermal energy accelerated oxygen diffusion into and oxidation of mPEG-co-PPyr NPs. Light absorption by the mPEG-co-PPyr NPs irradiated with a NIR laser (700 nm) increases the thermal energy. The resulting accelerated oxygen diffusion and oxidation might form bubbles on the surface the mPEG-co-PPyr NPs. Bubbles generated on the NPs provide the ultimate ultrasound-receiving signal, induced by laser irradiation. Further, thermal degradation of the mPEG-co-PPyr NPs might induce nonlinear elastic shrinking, fragmentation, and expansion, which can be detected as an ultrasound-receiving signal. The photoacoustic activity of the MoS2-incorporated NCs with mPEG-co-PPyr (mPEG-coPPyr/MoS2) was investigated (Fig. 4). The amplitude of the photoacoustic signal of mPEG-coPPyr/MoS2 was measured with variation of the MoS2 and mPEG-co-PPyr concentration (Fig. 4a). At a constant concentration of mPEG-co-PPyr (200 µg·ml-1), the photoacoustic signal amplitude was enhanced from 74.3 to 101.1 with increasing MoS2 incorporation from 10 to 100 µg·ml-1. In


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contrast, the photoacoustic signal enhancement induced by MoS2 complexation was not continuously augmented with a constant concentration of mPEG-co-PPyr (500 µg·ml-1). The photoacoustic signal declined at mass concentrations of MoS2 of 50 and 100 µg·ml-1. It is conjectured that the synergistic photoacoustic enhancement was restrained by a high concentration of MoS2 since the MoS2 nanosheets with high mass might be rumpled and condensed within the mPEG-co-PPyr complexes. Reduced heterogeneous contact within the complexes might deteriorate the intrinsic optical properties of MoS2 and thus that of the mPEGco-PPyr/MoS2 complexes. Supporting this result, the signal amplitude of mPEG-co-PPyr (200 µg·ml-1)/MoS2 was not significantly augmented (within 10% PA signal amplitude) with an increase of the MoS2 content to 10 µg·ml-1. In contrast, it was noted by that mPEG-coPPyr/MoS2 nanocomposites with only 10-20 µg of MoS2 addition amplified the PA signal. This concentration of 10-20 µg of MoS2 was lower than that of water dispersible individual PEGylated MoS2 nanosheets in water, reported in other studies. 18 The photoacoustic signal amplitude of the mPEG-co-PPyr/MoS2 NCs was measured with variation of the mPEG-co-PPyr concentration at a constant mass (20 µg·ml-1) of MoS2 (Fig. 4b). With the addition of a constant amount of MoS2 (20 µg·ml-1), the PA signal amplitude was significantly enhanced with an increase of the mPEG-co-PPyr concentration from 100 to 500 µg·ml-1. The PA signal increased logarithmically (R2 = 0.92), indicating that the combination of MoS2 and mPEG-co-PPyr was definitively characterized by cooperative interaction upon photoirradiation. One possible explanation for the PA synergistic effect in the mPEG-co-PPyr/MoS2 NCs is that appropriate heterogeneous contact between mPEG-co-PPyr and MoS2 enhances the heat conversion efficiency, heat conductance, and heat capacity. The thermal conductivity of two-dimensional transition metal disulfides was found to be dependent on the supporting



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materials and the charge mobility was enhanced with increased temperature, as reported in previous studies.28-29

Since the thermal conductivity of MoS2 nanosheets is two-orders of

magnitude lower than that of graphene,30-31 modulation of these nanosheets with a metal-like conductive polymer (herein, polypyrrole) was necessary for achieving high PA activity with MoS2. The molar extinction coefficient of polypyrrole/metal nanocomposites is higher than that of the polypyrrole NPs only, resulting in a stronger photothermal effect, as previously reported.12 It was suggested that the photothermal effect of polypyrrole/metal nanocomposites is mainly attributed to the intrinsic molar extinction, rather than the photothermal transduction efficacy, which is the energy conversion capacity, determined by the composite supramolecular structure. The mPEGco-PPyr/MoS2 NCs developed herein also had higher extinction coefficients for light absorbance compared to mPEG-co-PPyr only. The synergistic photoacoustic effects in the mPEG-coPPyr/MoS2 NCs might be induced by the high light absorption upon laser irradiation and the higher thermal energy might then be converted to acoustic waves. Furthermore, the heterogeneous surface contact of mPEG-co-PPyr and MoS2 might increase the synergistic enhancement of the photoacoustic signal activity. Several interlayers of MoS2 within mPEG-coPPyr NCs might be inflated by thermal ablation from laser irradiation, enhancing the PA signal amplitude.32 MoS2 and MoS2/metal NCs have been reported as biocompatible materials with lower cytotoxicity than other TMDs presented in recent studies, though extension into biomedical application requires further examination.18,

23, 32

Based on its unique optical properties and

propensity for conversion of thermal input into photoacoustic signals, the metal transition


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disulfide, MoS2, integrated with the π-conjugating polymer, mPEG-co-PPyr, represents a prospective emerging material as a platform for further investigation of the interface between versatile π-conjugating polymers and TMDs. The photoacoustic signal activity of mPEG-co-PPyr NPs and mPEG-co-PPyr/MoS2 NCs was investigated. Semisoluble mPEG-co-PPyr was synthesized to prepare surfactant-free SPNPs and SP/TMD NCs. The hybrid NCs of in mPEG-co-PPyr/MoS2 were examined to achieve enhancement of the photoacoustic effect. Methoxy PEG-co-PPyr NPs prepared by solvent exchange nanoprecipitation showed an absorption band in the range of 300 to 350 nm, corresponding to the π*-π transition of polypyrrole within the NPs. The optical properties of the two-dimensional MoS2 nanosheets were preserved in the mPEG-co-PPyr/MoS2 composites, indicating that complexation with mPEG-co-PPyr did not impair the intrinsic optical properties of MoS2. The mPEG-co-PPyr NPs and mPEG-co-PPyr/MoS2 nanocomposites both showed photoacoustic signal activity. The mPEG-co-PPyr/MoS2 NCs (10:1 in weight ratio) at even addition of 20 µg of MoS2 had significantly high photoacoustic signal amplitude, compared to the mPEG-co-PPyr NPs. The enhanced photoacoustic activity of the mPEG-co-PPyr/MoS2 NCs might be attributed to the heterogeneous interface contact between mPEG-co-PPyr and MoS2 upon complex formation. Laser ablation of MoS2 might elevate the local temperature and facilitate the thermal conductive transfer in the mPEG-co-PPyr/MoS2 NCs, amplifying PA signal. For the first time, mPEG-co-PPyr/MoS2 NCs comprising a semiconductive polymer and transition metal disulfide, have afforded the potential photoacoustic nanoplatforms for biosensorbased materials, biomedical theranostic application, and in the bio/electric interfacial field. Acknowledgements



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This

work

was

supported

by the Ministry of Science, ICT & Future Planning

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(NRF-

2015R1C1A2A01053307), the Korea Health Industry Development Institute (KHIDI), the Ministry of Health & Welfare (grant number: HI15C2797), and the National Research Foundation of Korea (NRF) (MSIP)(2014R1A2A2A03004531), Republic of Korea. Supporting Information Experimental methods of synthesis of mPEG-co-PPyr, Preparation methods of nanocomposites and the set-up of PA measurements, and Raman spectra of MoS2, mPEG-co-PPyr, and mPEGcoPPyr/MoS2.

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Figure Captions Scheme 1. Schematic of synthesis of pyrrole-terminated mPEG. Hydroxyl end group of mPEG was activated with butyl lithium and reacted with 2-(trichloroacetyl)pyrrole to prepare pyrroleterminated mPEG. Scheme 2. Schematic of mPEG-PPyr/MoS2 nanocomposites and generation of their photoacoustic signal. Figure 1. H1 NMR spectrum of pyrrole-terminated mPEG. Inset figure shows magnification of the 6 to 10 ppm range to enlarge pyrrole signals. Figure 2. Characteristics of mPEG-co-PPyr nanoparticles and mPEG-co-PPyr/MoS2 nanocomposites: (a) TEM images of MoS2, mPEG-co-PPyr, mPEG-co-PPyr/MoS2 were presented in the order of raw. The second and third columns present its magnified images. DLS and zeta potential data (b) and UV-vis absorbance spectra (c) of mPEG-co-PPyr and mPEG-coPPyr/MoS2. Methoxy PEG-co-PPyr/MoS2 NCs were prepared with weight ratio of 200/20 µg per 1 ml of DI water and diluted for the measurements. All measurements of diameter, zeta potentials, and UV absorbance were measured in DI water with adjustment of pH 7.0. Figure 3. Figure 3. Concentration dependence of mPEG-co-PPyr nanoparticles for photoacoustic activity at high and low laser energy. The high average energy density of the laser irradiation was 14.48 mJ·cm-2. Inner and outer diameter of used Tygon tubes were ID 0.05 in, OD 0.09 Figure 4. Photoacoustic activity of mPEG-co-PPyr nanoparticles and mPEG-co-PPyr/MoS2 nanocomposites. (a) PA signal of mPEG-co-PPyr/MoS2 NCs with varied MoS2 concentrations (0 to 100 µg), and (b) PA signal of mPEG-co-PPyr/MoS2 NCs at constant MoS2 concentration (20 µg) and with varied concentration of mPEG-co-PPyr (100-500 µg). All mPEG-co-PPyr/MoS2 NCs complexed with MoS2 showed significantly enhanced photoacoustic activity. The average energy density of the laser irradiation was 14.48 mJ·cm-2 in each experiment of (a) and (b). Inner and outer diameter of used tygon tubes were ID 0.05 in, OD 0.09 in and ID 0.04 in, OD 0.07 in, respectively for experiment (a) and (b).


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1) BuLi, THF, -78oC

O H3CO

nH

o

2) 2-(trichloroacetyl)pyrrole, -20 C

Fe(PTS) 3, MC, 20oC

O H3CO

n

O

N H

H3 CO

n

O

N H

n

O

N H

N H

N H

N H

N H

or

m

H N

H N

H N

O H 3CO

H N

H N

O

O O

CH 3 n

m

Scheme 1. Schematic of synthesis of pyrrole-terminated mPEG. Hydroxyl end group of mPEG was activated with butyl lithium and reacted with 2-(trichloroacetyl)pyrrole to prepare pyrroleterminated mPEG.

Scheme 2. Schematic of mPEG-co-PPyr/MoS2 nanocomposites and generation of their photoacoustic signal.



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Figure 1. H1 NMR spectrum of pyrrole-terminated mPEG. Inset figure shows magnification of the 6 to 10 ppm range to enlarge pyrrole signals.


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Figure 2. Characteristics of mPEG-co-PPyr nanoparticles and mPEG-co-PPyr/MoS2 nanocomposites: (a) TEM images of MoS2, mPEG-co-PPyr, mPEG-co-PPyr/MoS2 were presented in the order of raw. The second and third columns present its magnified images. DLS and zeta potential data (b) and UV-vis absorbance spectra (c) of mPEG-co-PPyr and mPEG-coPPyr/MoS2. Methoxy PEG-co-PPyr/MoS2 NCs were prepared with weight ratio of 200/20 µg per 1 ml of DI water and diluted for the measurements. All measurements of diameter, zeta potentials, and UV absorbance were measured in DI water with adjustment of pH 7.0.



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Figure 3. Concentration dependence of mPEG-co-PPyr nanoparticles for photoacoustic activity at high and low laser energy. The high average energy density of the laser irradiation was 14.48 mJ·cm-2. Inner and outer diameter of used Tygon tubes were ID 0.05 in, OD 0.09


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Figure 4. Photoacoustic activity of mPEG-co-PPyr nanoparticles and mPEG-co-PPyr/MoS2 nanocomposites. (a) PA signal of mPEG-co-PPyr/MoS2 NCs with varied MoS2 concentrations (0 to 100 µg), and (b) PA signal of mPEG-co-PPyr/MoS2 NCs at constant MoS2 concentration (20 µg) and with varied concentration of mPEG-co-PPyr (100-500 µg). All mPEG-co-PPyr/MoS2 NCs complexed with MoS2 showed significantly enhanced photoacoustic activity. The average energy density of the laser irradiation was 14.48 mJ·cm-2 in each experiment of (a) and (b). Inner and outer diameter of used tygon tubes were ID 0.05 in, OD 0.09 in and ID 0.04 in, OD 0.07 in, respectively for experiment (a) and (b).



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Methoxy Polyethylene

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