Self-assembly of micro- and nano-scale particles using bio-inspired events
Introduction
While engineers and scientist are aspiring to controllably manipulate structures at the micro- and nano-meter scale, nature has been performing these tasks and assembling complex structures with great accuracy and high efficiency using highly specific molecules such as DNA and proteins. Since Mirkin et al. [1] and Aliviastos et al. [2] demonstrated DNA mediated-assembly of gold nanoparticles; there has been a tremendous interest and many reports in the use of DNA to specifically assemble micro- and nano-size particles for chemical and biological detection and for possible uses in electronic circuitry to assemble future nano-electronic devices. Numerous approaches to the challenge of assembly have been demonstrated to include biological entity mediated [3], [4], chemically mediated [5], electrically mediated [6], and fluidics mediated [7]. Patterned assembly of micron-sized Au disk using electrostatic forces has been demonstrated [8]. However, these were not active devices, rather passive Au pads that were not suitable for any electronic applications. Active devices such as carbon nanotubes [9] and InGaAs LEDs [10] as well as metallic nanorods [11] for interconnect systems have been assembled. Additional work is required to bring these systems to application.
Nature assembles nano-scale components using molecular recognition. In the case of DNA, hydrogen bonding provides the specificity behind the matching of complementary pairs of single-stranded (ss) DNA to hybridize into a double strand (ds) of helical DNA. It has been estimated that each base pair binds with 0.5 kcal/mol of energy [12]. For the use of a 18mer oligonucleotide, the binding energy can be estimated at 9 kcal/mol. The actual binding energy of a dsDNA is dependent on the base-pair sequence, salt concentration of surrounding media, temperature, among others. In the case of antibodies/antigens and ligands/receptors, binding takes places by a combination of electrostatic forces, chemical bonding, and shape-mediated effects. As an example, avidin is a large protein that has binding sites for four biotin molecules. The affinity of the biotin–avidin complex is equivalent to 21 kcal/mol [13]. In comparison, the Au–S thiolate covalent bond, which is used to attach thiol conjugated oligonucleotides to gold surfaces has an energy of 44 kcal/mol [14]. Although, taken individually, these energies are considerably less than that for many covalent single bonds (e.g. the C–O bond is 96 kcal/mol), in composite, they are sufficiently strong to provide stable attachment at ambient temperatures.
In this study, the assembly of micro- and nano-scale particles using biologically inspired events such as DNA hybridization and ligand/receptor interactions has been investigated. Different approaches can be taken to particle assembly, as shown in Fig. 1(a-c). For all three cases, the particles could be beads, devices, or other nano and micro-scale objects. Fig. 1(a) presents an approach that uses DNA only and relies on hybridization for attachment of the islands to the surface [15]. The substrate surface is functionalized with ssDNA while the complementary strand is attached to the gold surface of the particle. Direct hybridization into a dsDNA would result in capture of the islands onto the patterned gold surface. Indirect hybridization has also been demonstrated, in which a third DNA strand is used to hybridize with the strands attached to the surface and to the particles. We have begun with the approach shown in Fig. 1(b), which utilizes DNA and ligand/receptors for the capture of avidin-coated particles. A dsDNA is attached to a surface presenting a biotin. The strong affinity between avidin and biotin is relied upon to ultimately capture the particle. This design allows for the verification of ssDNA attachment to the surface, hybridization into the dsDNA and the availability of biotin for particle capture. Fig. 1(c) also shows the capture of particles with a thin layer of gold so the particles may also be functionalized with DNA via the Au–S bond. The ssDNA is attached to the gold surface and subsequently hybridized just as the patterned surface is functionalized. Again, the dsDNA present biotin molecules for attachment to the avidin molecule. Avidin is first absorbed on the substrate surface and then the particles are captured using one of the other three sites of the immobilized avidin. We have used the approach depicted in Fig. 1(c) for the attachment of devices to each other in a fluid.
Section snippets
Materials and procedures
Variations in the biological entities and labeling, and the sequence of surface and particle functionalization were used during experimentation which are described below.
Polystyrene bead assembly and optimization
The assembly of avidin-coated 0.8 μm diameter polystyrene beads [19] onto patterned substrates was initially investigated due to the availability of various sizes and labels in addition to the high concentrations and low mass. The substrates functionalized with dsDNA presenting a biotin molecule were placed in a solution of avidin-coated beads and PBS. The avidin–biotin reaction was performed for 45 min at 37 °C while swirling the vial contents. Rinsing the sample under a stream of DI water was
Further discussion
The assembly of micro- and nano-scale particles onto patterned surfaces using bio-inspired events is a significant challenge. Forces involved in assembly have included covalent, biological/molecular, electrostatic, gravity, and viscous drag. To complete the particle assembly for useful electronics, the substrates must be rinsed of non-specifically attached particles and dried. Thus, the forces of assembly must be greater than the forces encountered during subsequent processing. The combined
Conclusions
This paper has presented our progress towards the biologically inspired assembly of micro- and nano-scale particles. ssDNA attachment to patterned surfaces and the subsequent hybridization of a complementary strand was demonstrated. Polystyrene beads, with 0.8 μm diameter and coated with avidin were captured and assembled on patterned gold surfaces functionalized with dsDNA with a biotin at the end. Streptavidin-coated gold nanoparticles were also captured using biotinylated-DNA on patterned
Acknowledgements
This work was supported by Center of Nanoscale Devices at Purdue University funded through the State of Indiana, 21st Century Research and Technology Fund. We would also like to acknowledge Prof. Steve Wereley for helpful discussions and the Micro-fabrication facility and Staff at Purdue University for assistance in device processing.
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