Janus and Amphiphilic MoS2 2D Sheets for Surface-Directed Orientational Assemblies toward Ex Vivo Dual Substrate Release

Abstract

The two-dimensional (2-D) Janus and amphiphilic molybdenum disulfide (MoS₂) nanosheet with opposite optical activities on each side (amphichiral) is synthesized by modifying sandwich-like bulk MoS₂ with tannic acid and cholesterol through biphasic emulsion method. This new type of amphichiral Janus MoS₂ nanosheet consists of a hydrophilic and positive optical activity tannic acid side as well as a hydrophobic and negative optical activity cholesterol side thereby characterized by circular dichroism. Surface-directed orientational differentiation assemblies are performed for the as-synthesized 2D material and are characterized by contact angle, infrared spectroscopy, X-ray photoelectron, and circular dichroism spectroscopies. The amphiphilic nature of the materials is demonstrated by the pre-organization of the nanosheets on either hydrophobic or hydrophilic surfaces, providing unprecedented properties of circular dichroism signal enhancement and wettability. Selective detachment of the surface organic groups (cholesterol and tannic acid fragments) is realized by matrix-assisted laser desorption/ionisation - time-of-flight (MALDI-TOF) mass spectrometry, and the dual substrate release in tissue is detected by ex vivo mass spectrometry imaging.

Keywords: 2‐D materials; janus materials; mass spectrometry imaging; molybdenum disulfide; surface‐directed orientational self‐assembly.

Figure 1

Preparation of tannic acid (TA)‐based hydrophilic (TA‐MoS₂‐TA), cholesterol (Chol)‐based hydrophobic (Chol‐MoS₂‐Chol) and TA/Chol‐based Janus amphiphilic (TA‐MoS₂‐Chol) 2D MoS₂ sheets.

Figure 2

TEM images of TA/Chol‐based Janus amphiphilic (TA‐MoS₂‐Chol) 2D MoS₂ sheets A–C) and AFM images of TA‐MoS₂‐Chol in water D) and toluene E) solution deposited on mica surface.

Figure 3

A) FTIR spectra of TA‐MoS₂‐Chol (Top), Chol‐MoS₂‐Chol (Middle), and TA‐MoS₂‐TA (Bottom) with observed absorption signals at 3500 cm⁻¹ (O─H stretching, grey), 3000 cm⁻¹ (C─H stretching, blue), 1750 cm⁻¹ (C═O stretching, pink), 1350 cm⁻¹ (C─O stretching, yellow), 750 cm⁻¹ (C─C bending, green) and 500 cm⁻¹ (Mo─S stretching, light blue). B) The chemical structures of the (+)tannic acid and (–)cholesterol, blue spot indicates the chiral carbon.

Figure 4

Surface‐directed orientational differentiation assemblies of Janus and amphiphilic TA‐MoS₂‐Chol assembled selectively on hydrophobic HOPG and hydrophilic MICA surfaces, revealing the hydrophilic TA groups on top and hydrophobic Chol groups on top, respectively.

Figure 5

Water contact angle measurements of TA‐MoS₂‐TA, TA‐MoS₂‐Chol, and Chol‐MoS₂‐Chol on HOPG A,C,E) and mica B,D,F). On HOPG, the amphiphilic TA‐MoS₂‐Chol has a wettability closer to TA‐MoS₂‐TA (Δ = 10.2°) instead of the Chol‐MoS₂‐Chol (Δ = 13.6°). On MICA, the amphiphilic TA‐MoS₂‐Chol has a wettability closer to Chol‐MoS₂‐Chol (Δ  = 13.9°) instead of the TA‐MoS₂‐TA (Δ = 14.9°). These results reveal the surface‐directed orientational differentiation assemblies of Janus and amphiphilic TA‐MoS₂‐Chol assembled selectively on hydrophobic HOPG and hydrophilic MICA surfaces.

Figure 6

XPS spectra of TA‐MoS₂‐TA, TA‐MoS₂‐Chol, and Chol‐MoS₂‐Chol on HOPG A,C,E) and MICA B,D,F), analyzing the C─C (red), C─O (blue), and average (green or black) signals with baseline correction (purple).

Figure 7

Absorption A,C,E) and circular dichroism B,D,F) spectra of TA‐MoS₂‐TA, TA‐MoS₂‐Chol, and Chol‐MoS₂‐Chol, respectively. Red arrows show the distinctive signals. TA‐MoS₂‐Chol revealed a combination of a positive signal at 205 nm and negative signal at 215 nm with signal enhancements.

Figure 8

MALDI‐TOF mass spectrometry of TA‐MoS₂‐Chol for profiling tannic acid (TA)‐derivatives (matrix‐free) and cholesterol (Chol)‐derivative (with DHB, HCCA, and SA matrixes).

Figure 9

MALDI‐TOF mass spectrometric imaging (MSI) of kidney tissues embedded with TA‐MoS₂‐TA, TA‐MoS₂‐Chol, and Chol‐MoS₂‐Chol for detection of simultaneously dual release of TA‐derivative and Chol ex vivo. A–C) Optical images of the tissues treated with TA‐MoS₂‐TA, TA‐MoS₂‐Chol, and Chol‐MoS₂‐Chol, respectively. D,E) MSI analyzed at m/z 124.9720 mapping to a TA fragment of TA‐MoS₂‐TA and TA‐MoS₂‐Chol, respectively. F,G) MSI analyzed at m/z 386.7490 profiling to a Chol derivative of TA‐MoS₂‐Chol, and Chol‐MoS₂‐Chol, respectively.