Abstract
Graphene quantum dots (GQDs) have received great attention as optical agents because of their low toxicity, stable photoluminescence (PL) in moderate pH solutions, and size-dependent optical properties. Although many synthetic routes have been proposed for producing GQD solutions, the broad size distribution in GQD solutions limits its use as an efficient optical agent. Here, we present a straightforward method for size fractionation of GQDs dispersed in water using a cross-flow filtration system and a track-etched membrane with cylindrical uniform nanopores. The GQD aqueous suspension, which primarily contained blue-emitting GQDs (B-GQDs) and green-emitting GQDs (G-GQDs), was introduced to the membrane in tangential flow and was fractionated with a constant permeate flow of about 800 L m−2 h−1 bar−1. After filtration, we observed a clear blue PL spectrum from the permeate side, which can be attributed to selective permeation of relatively small B-GQDs. The process provided a separation factor (B-GQDs/G-GQDs) of 0.74. In the cross-flow filtration system, size-dependent permeation through cylindrical nanochannels was confirmed by simulation. Our results demonstrate a feasible method facilitating size fractionation of two-dimensional nanostructures using a cross-flow membrane filtration system. Since membrane filtration is simple, cost-effective, and scalable, our approach can be applied to prepare a large amount of size-controlled GQDs required for high performance opto-electronics and bio-imaging applications.
Highlights
Graphene quantum dots (GQDs) are nano-sized monolayers or few-layer graphene sheets with a two-dimensional hexagonal lattice structure [1,2,3]
GQDs exhibit quantum confinement because the size of the system is comparable with the de Broglie wavelength of an electron, which is not present in bulk graphene [2]
As quantum confinement is dependent on the size of the system [4,5,6,7], GQDs generate different photoluminescence (PL) spectra as a function of the size of the GQD; this is well-known from theoretical predictions and experimental results [8,9]
Summary
Graphene quantum dots (GQDs) are nano-sized monolayers or few-layer graphene sheets (below 20 nm) with a two-dimensional hexagonal lattice structure [1,2,3]. GQDs smaller than 10 nanometers pass cell membranes and are very stable without photo-bleaching [18] They exhibit better biocompatibility compared with other nano-sized imaging agents [15,19]. Components larger than membrane pores are retained and pass along the membrane surface, flowing back to the feed reservoir This filtration mode could minimize membrane fouling and provides a stable flux compared to dead-end filtration [35,36]. Selective permeation of relatively small GQDs through the membrane occurred for a given pair of cross-flow velocity and pore size values; this result was confirmed by comparing with simulation results. We evaluated the feasibility of the crossflow membrane system for size fractionation of GQDs by characterizing the selectivity and permeability in membrane filtration, and the fouling on membrane surfaces by comparing with a conventional dead-end filtration system
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