Scale‐Up Synthesis of Fragrant Nitrogen‐Doped Carbon Dots from Bee Pollens for Bioimaging and Catalysis

Abstract

In view of the repeatability and scalability of the synthesis, we substituted natural pollen with commercial bee pollens.Bee pollen is a mass of pollen packed by worker honeybees into granules used for feeding its larvae in the early stage of development and producing royal jelly.For centuries bee pollens have been favorably consumed as diet supplementation since they are rich in biomolecules, including sugars, proteins, amino acids, and nucleic acids.[ 9 ] There are several noteworthy merits in producing CDs from bee pollens.First, the variety of bee pollens is abundant in markets and the price can be affordable.For the bee pollens used in this study, each bottle of bee pollen weighted 500 g costs 35 Chinese yuan on average, thus every 1 g costing only 0.07 Chinese yuan, or 0.011 dollar.Second, the synthesis is green, remarkably simple, highly reproducible, and capable to be scaled up to large volumes.Third, the production yield is fairly high.At least 3 g of CDs can be prepared from 10 g input of the raw material, manifesting a conversion ratio of ≈30% (w/w).Fourth, the CDs are fragrant and doped with a relatively high content of nitrogen, exhibiting fl uorescence quantum yields (QYs) between 6.1% and 12.8%, depending on the species of bee pollen and reaction time.Fifth, the CDs display excellent biocompatibility.Last, with their sizes being 1-2 nm, the dots manifest superior catalysis and photocatalysis towards the reduction of noble metal ions in aqueous solution.Three kinds of bee pollens were utilized in our study, namely rapeseed fl ower bee pollen, camellia bee pollen, and lotus bee pollen (Figure S1, Supporting Information).To be simplifi ed, CDs derived from the three bee pollens are individually denoted as r-CDs, c-CDs, and l-CDs, with respect to rapeseed fl ower, camellia, and lotus, respectively.Figure 1 a shows the optical absorption spectrum of diluted solution of r-CDs after hydrothermal treatment at 180 °C for 24 h.The elemental analysis (Table S2, Supporting Information) shows that the content percentage of carbon in the fi nal product increases while that of oxygen minimizes relative to those in the bee pollen, providing defi nitive evidence for the carbonization of the molecule precursors within.This is refl ected by a well-defi ned absorption peak centered at 280 nm consistent with the representative π-π* transition of aromatic carbon.Moreover, there is an appreciable increase of nitrogen content in the CDs, suggesting the doping of nitrogen in the carbon structure during condensation reactions.In vivid contrast, the yellow turbid suspension of bee pollen changes to brown transparent solution of CDs after purifi cation, as shown in Figure 1 c.Like many other CDs, [ 2a , 4a , 5e , 10 ] the fl uorescence emission of the r-CDs is excitation wavelength dependent, tunable from 425 to 505 nm upon excitation from 340 to 450 nm, enabling the observation of visible color shift from blue through cyan to green (Figure 1 b,d).The fl uorescence QY upon excitation at 380 nm was calculated to be 9.1%, using quinine sulfate as the reference.