Herein, we report on the preparation, purification, and preliminary characterization of multicolor fluorescent carbon nanoparticles (CNPs) obtained from the combustion soot of candles. The CNPs are small (< 2 nm) and water soluble. Different CNPs fluoresce with different colors under a single-wavelength UV excitation. Carbon-based nanomaterials, which include carbon nanotubes, fullerenes, and nanofibers, have promising applications in nanotechnology, biosensing, and drug delivery.[1–3] Recently, CNPs—a new class of carbon-based nanomaterials with interesting photoluminescence properties—were isolated.[4–10] These nanoparticles are either nanodiamonds or materials derived from carbon nanotubes and the laser ablation of graphite. Unlike fluorescent semiconductor nanocrystals (so-called quantum dots or Qdots), the fluorescent CNPs have only been poorly studied up to now because of the lack of preparative methods and separation techniques. Herein, we report a method for efficiently preparing and isolating fluorescent CNPs from a common carbon source, namely, candle soot.

Our approach includes: 1) The preparation of fluorescent CNPs from the combustion soot of candles by means of an oxidative acid treatment and 2) the purification of the fluorescent CNPs by using polyacrylamide gel electrophoresis (PAGE). Incomplete combustion produces CNPs with diameters of 20–800 nm.[11, 12] These particles strongly interact with each other to form agglomerates of several micrometers. To break down such inherent interactions and produce welldispersed, individual CNPs, we adopted an oxidative acid treatment, which is commonly used for the purification of carbon nanotubes.[13] This method is known to introduce OH and CO2H groups to the CNP surfaces,[14] thus making the particles become negatively charged and hydrophilic. The candle soot was collected by sitting a glass plate on top of smoldering candles. The soot contained mainly elemental carbon (elemental analysis: C 91.69%, H 1.75%, N 0.12%, O (calculated) 4.36%) and was hydrophobic and insoluble in common solvents. After refluxing the candle soot with 5m HNO3, it turned into a homogeneous, black aqueous suspension. Upon centrifugation, the suspension separated into a black carbon precipitate and a light-brown supernatant,