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][2][3] Recently, CNPs-a new class of carbon-based nanomaterials with interesting photoluminescence properties-were isolated. [4][5][6][7][8][9][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 CO 2 H 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 5 m HNO 3 , it turned into a homogeneous, black aqueous suspension. Upon centrifugation, the suspension separated into a black carbon precipitate and a light-brown supernatant, which exhibited yellow fluorescence when irradiated with UV light (312 nm). The black precipitate also contained fluorescent material (even after washing it several times). For maximum recovery of this fluorescent material, both the supernatant and the precipitate were neutralized and then extensively dialyzed against water. The neutralized candle soot exhibited an excellent dispersibility in water, which lasted several months.The same procedure failed to generate visible fluorescence if an oxidant, such as HNO 3 , was not present (this happened both in the presence and in the absence of surfactants, SDS). Another oxidant (30 % H 2 O 2 /AcOH = 2:1) resulted in blue fluorescence. The oxidative acid treatment might have three functions:...

DNA is renowned for its double helix structure and the base pairing that enables the recognition and highly selective binding of complementary DNA strands. These features, and the ability to create DNA strands with any desired sequence of bases, have led to the use of DNA rationally to design various nanostructures and even execute molecular computations. Of the wide range of self-assembled DNA nanostructures reported, most are one- or two-dimensional. Examples of three-dimensional DNA structures include cubes, truncated octahedra, octohedra and tetrahedra, which are all comprised of many different DNA strands with unique sequences. When aiming for large structures, the need to synthesize large numbers (hundreds) of unique DNA strands poses a challenging design problem. Here, we demonstrate a simple solution to this problem: the design of basic DNA building units in such a way that many copies of identical units assemble into larger three-dimensional structures. We test this hierarchical self-assembly concept with DNA molecules that form three-point-star motifs, or tiles. By controlling the flexibility and concentration of the tiles, the one-pot assembly yields tetrahedra, dodecahedra or buckyballs that are tens of nanometres in size and comprised of four, twenty or sixty individual tiles, respectively. We expect that our assembly strategy can be adapted to allow the fabrication of a range of relatively complex three-dimensional structures.

We live in a macroscopic three-dimensional world, but our best description of the structure of matter is at the atomic and molecular scale. Understanding the relationship between the two scales requires that we bridge from the molecular world to the macroscopic world. Connecting these two domains with atomic precision is a central goal of the natural sciences, but it requires high spatial control of the 3D structure of matter.1 The simplest practical route to producing precisely designed 3D macroscopic objects is to form a crystalline arrangement by self-assembly, because such a periodic array has only conceptually simple requirements: [1] A motif whose 3D structure is robust, [2] dominant affinity interactions between parts of the motif when it self-associates, and [3] a predictable structures for these affinity interactions. Fulfilling all these criteria to produce a 3D periodic system is not easy, but it should readily be achieved by well-structured branched DNA motifs tailed by sticky ends.2 Complementary sticky ends associate with each other preferentially and assume the well-known B-DNA structure when they do so;3 the helically repeating nature of DNA facilitates the construction of a periodic array. It is key that the directions of propagation associated with the sticky ends not share the same plane, but extend to form a 3D arrangement of matter. Here, we report the crystal structure at 4 Å resolution of a designed, self-assembled, 3D crystal based on the DNA tensegrity triangle.4 The data demonstrate clearly that it is possible to design and self-assemble a well-ordered macromolecular 3D crystalline lattice with precise control.

The assembly of synthetic, controllable molecular mechanical systems is one of the goals of nanotechnology. Protein-based molecular machines, often driven by an energy source such as ATP, are abundant in biology. It has been shown previously that branched motifs of DNA can provide components for the assembly of nanoscale objects, links and arrays. Here we show that such structures can also provide the basis for dynamic assemblies: switchable molecular machines. We have constructed a supramolecular device consisting of two rigid DNA 'double-crossover' (DX) molecules connected by 4.5 double-helical turns. One domain of each DX molecule is attached to the connecting helix. To effect switchable motion in this assembly, we use the transition between the B and Z forms of DNA. In conditions that favour B-DNA, the two unconnected domains of the DX molecules lie on the same side of the central helix. In Z-DNA-promoting conditions, however, these domains switch to opposite sides of the helix. This relative repositioning is detected by means of fluorescence resonance energy transfer spectroscopy, which measures the relative proximity of two dye molecules attached to the free ends of the DX molecules. The switching event induces atomic displacements of 20-60 A.

Recent work has demonstrated the self-assembly of designed periodic two-dimensional arrays composed of DNA tiles, in which the intermolecular contacts are directed by 'sticky' ends. In a mathematical context, aperiodic mosaics may be formed by the self-assembly of 'Wang' tiles, a process that emulates the operation of a Turing machine. Macroscopic self-assembly has been used to perform computations; there is also a logical equivalence between DNA sticky ends and Wang tile edges. This suggests that the self-assembly of DNA-based tiles could be used to perform DNA-based computation. Algorithmic aperiodic self-assembly requires greater fidelity than periodic self-assembly, because correct tiles must compete with partially correct tiles. Here we report a one-dimensional algorithmic self-assembly of DNA triple-crossover molecules that can be used to execute four steps of a logical (cumulative XOR) operation on a string of binary bits.

A two-dimensional DNA crystal has been designed and constructed from Holliday junction analogues that contain two helical domains twisted relative to each other. The Holliday junction is not an inherently rigid system, but it can be made less flexible if it is combined into a larger construct. We have fused four junctions into a rhombus-like molecule consisting of four six-turn helices, two on an upper layer and two on a lower layer; the branch points, which define vertices, are separated by four double helical turns each. Ligation of the rhombus-like motifs produces no cyclic species, when assayed by ligation-closure experiments. Self-assembly of the rhombuses in one dimension leads to a linear pattern. The rhombuses can be directed to self-assemble by hydrogen bonding into a two-dimensional periodic array, whose spacing is six turns in each direction. The expected spacing is seen when the array is observed by atomic force microscopy (AFM). Variation of the dimensions of the repeat unit from six turns × six turns to six turns × eight turns results in the expected increase in unit cell dimensions. Hence, it is possible to assemble periodic arrays with tunable cavities using these components. This system also provides the opportunity to measure directly the angles or torsion angles between the arms of branched junctions; here we measure the torsion angle between the helical domains of the Holliday junction analogue. We find by AFM that the torsion angle between helices is 63.5°, in good agreement with previous estimates.

A three-point-star DNA motif has been designed and constructed, which can self-assemble into hexagonal two-dimensional lattices. The resulting lattices are up to 1 mm.

Structural DNA nanotechnology and the DNA origami technique, in particular, have provided a range of spatially addressable two- and three-dimensional nanostructures. These structures are, however, typically formed of tightly packed parallel helices. The development of wireframe structures should allow the creation of novel designs with unique functionalities, but engineering complex wireframe architectures with arbitrarily designed connections between selected vertices in three-dimensional space remains a challenge. Here, we report a design strategy for fabricating finite-size wireframe DNA nanostructures with high complexity and programmability. In our approach, the vertices are represented by n × 4 multi-arm junctions (n = 2-10) with controlled angles, and the lines are represented by antiparallel DNA crossover tiles of variable lengths. Scaffold strands are used to integrate the vertices and lines into fully assembled structures displaying intricate architectures. To demonstrate the versatility of the technique, a series of two-dimensional designs including quasi-crystalline patterns and curvilinear arrays or variable curvatures, and three-dimensional designs including a complex snub cube and a reconfigurable Archimedean solid were constructed.