Close-Packed Langmuir Monolayers of Saccharide-Based Carbon Dots at the Air–Subphase Interface

Abstract

Carbon dots (CDs) are zero-dimensional carbonbased spherical nanoparticles with diameters less than 10 nm. Here, we report for the first time CDs forming stable Langmuir monolayers at the air−subphase interface. Langmuir monolayers are of great interest both fundamentally to study the interactions at the interfaces and for many applications such as the development of sensors. However, CDs usually do not form Langmuir monolayers because of their highly hydrophilic nature. In this study, amphiphilic CDs were prepared through hydrothermal carbonization using saccharides as the precursors. The surface chemistry behavior and optical properties of CDs at the air− subphase interface were studied. CDs derived from saccharides consistently formed stable Langmuir monolayers which show all essential phases, namely, gas, liquid-expanded, liquid-condensed, and solid phases. The compression−decompression cycle method showed minimum hysteresis (4.3%), confirming the retaining capacity of the CDs as a monolayer. Limiting CD areas from surface pressure−area isotherm at the air−subphase interface were used to calculate the average diameter of the CDs at the air−subphase interface. UV/vis absorption spectra of CDs dispersed in water and in Langmuir monolayers had the same bands in the UV region. The intensity of the UV/vis absorption increases with increasing surface pressure at the air−subphase interface. Interestingly, photoluminescence (PL) of the Langmuir monolayer of CDs was excitation-independent, whereas the same CDs had excitation-dependent PL when dispersed in water.

Carbon Dots: A Future Blood–Brain Barrier Penetrating Nanomedicine and Drug Nanocarrier

“…For instance, Zhao et al used benzenetetramine to prepare CDs with emission wavelength above 600 nm. 19 A variety of precursors have been used to synthesize CDs hydrothermally or solvothermally, such as carbohydrates, 20 proteins, 21,22 and many organic acids including amino acids and citric acids. [23][24][25][26] Meanwhile, ultrasonication represents a gentler method compared with microwave irradiation or hydrothermal/solvothermal approaches, although examples of this technique are rare.…”

Crossing the blood–brain barrier with carbon dots: uptake mechanism andin vivocargo delivery

“…Synthesis and characterization of glucose-based carbon dots and uorescein conjugate GluCDs were synthesized using the same methodology reported in our previous work. 25 Fig. 2a shows a schematic representation of the synthesis of GluCDs with glucose as the sole precursor via a hydrothermal bottom-up method and used for in vitro studies.…”

“…2b is a schematic representation of the conjugation of GluCDs to 5-(aminomethyl)-fluorescein via EDC/NHS amidation coupling. As GluCDs have carboxylic acid surface functional groups 25 and 5-(aminomethyl)-fluorescein has a primary amine group, they form a stable amide bond through the EDC/NHS conjugation reaction.…”

“…The band around 260–325 nm can be attributed to the π–π* transition (C C) whereas the band around 325–400 nm can be attributed to the n–π* transition (C O). 25 The peak in the GluCD-F's absorption spectrum at around 492 nm stems from fluorescein. Fig.…”

“…GluCDs were prepared with d -glucose (VWR International, LLC; Radnor, PA) as the only precursor using the same method we previously reported. 25 Briefly, 20 ml of 0.3 M d -glucose solution was prepared in DI-water obtained from Direct-Q 3 water purification system (EMD Millipore Corp.; Chicago, IL) with a resistivity of 18 MΩ cm at 20.0 ± 0.5 °C and surface tension of 71.2 mN m −1 at the same temperature. The solution was heated up to 200 °C in 30 min and kept at 200 °C for 5 h in a Teflon-coated autoclave reactor (DW-5MB2-BG61, BAOSHISHAN, China) using a muffle furnace (BF51800 Series; Thermo Scientific; Rockford, IL).…”