Plasmonic Multilayers based on DNA-coated gold nanoparticles

Gold nanoparticles coated with single-stranded DNA ligands have a powerful dual functionality: The DNA ligands can be engineered with highly specific end groups that selectively attach to DNA strands on the surfaces of viruses and bacteria, or to detect microRNAs in the bloodstream that are early cancer indicators. The gold core serves as a light amplifier via the surface plasmon resonance for detection of agents using for instance surface-enhanced Raman spectroscopy.

In order to increase the sensitivity of such biosensors, CHESS scientists Tom Derrien and Detlef Smilgies designed a porous multilayer built of these nanoparticles. They were joined by postdoc Shogo Hamada and undergraduate student Max Zhou in Dan Luo's lab at Cornell's Department of Biological and Environmental Engineering where the DNA-coated nanocrystals were prepared. Their results were recently published in high-profile journal Nano Today [1].


plasmonic mutilayers

Figure 1. Plasmonic multilayers build with DNA-coated gold nanoparticles. The first layer was attached to the silicon sunstrate using the positively chanrged amine group of APTES, Successive layers of DNA-coated nanoparticles were formed by dipping the film into a solution of poly-L-lysine (PLL), a polymer of the essential amino acide L-lysine which has an additional amine group that compensates the negative charge of the DNA backbones of the gold particles in the adjacent layers.


The challenge in building multilayers lies in the fact that in aqueous medium the phosphate groups of the DNA backbone are negatively charged. In an early study the group had succeeded in creating dense monolayers of the nanoparticles by functionalizing the glass substrate with a molecule call APTES (amino propyl triethoxy silane) [1]. The APTES layer reacts with the glass surface forming a firm chemical bond. In aqueous medium the amine group at the other end of the molecule is fully protonated and thus positively charged attracting the negative DNA ligands. However due to the residual negative charge no nanoparticles attach any more to the substrate as soon as the monolayer is filled. The scientists had to find another "glue"; this came in form of the chain molecule poly-L-lysine which consists of many units of L-lysine, an essential amino acid which has an extra amine group that is positively charged. By alternatively dipping the glass substrate in poly-L-lysine solution and DNA-coated nanoparticle solution, sandwiches of two to ten nanoparticle layers could be built up. This approach was origionally developed by Gero Decher [3] and coined "molecular beaker epitaxy". The Cornell study applied the idea to such highly functionalizable nanoparticles.



plasmonic properties

Figure 2. Colors of plasmonic multilayers of DNA coated gold nanoparticles in transmission  (top) and reflection (bottom). The upper value displays the core diameter and the lower value the number of layers. In each case ligands with 60 base pairs were used. Particles were deposited from solution onto a glass slide using a stencil.


X-ray scattering results at D1 beamline showed that particles were closely spaced, as expected by the number of base pairs of the DNA ligands and the salt concentration of the solutions. Optical spectroscopy demonstrated the plasmonic nature of this novel multilayer system. The optical absorption depended on the diameter of the gold core, the number of layers and the ligand length. Effects are most pronounced in the first few layers. In order to demonstrate the optical variability of the material to the naked eye Tom Derrien deposited different particles and layer numbers on a stencil and took pictures in transmission and reflection (see Figure 2). The multilayer system shows interesting optial tunability which could help to tune light to the color range where detection of biological agents is optimized.


Submitted by Detlef Smilgies

References

[1] Thomas L. Derrien, Shogo Hamada, Max Zhou, Detlef-M. Smilgies, and Dan Luo: "Three-dimensional nanoparticle assemblies with tunable plasmonics via a layer-by-layer process", Nano Today 30 (2020) 100823. 


[2] Thomas L. Derrien, Michelle Zhang, Patrick Dorion, Detlef-M. Smilgies, and Dan Luo: "Assembly Dynamics of Plasmonic DNA-capped Gold Nanoparticle Monolayers", Langmuir 2018, 34, 14711–14720.

[3] G. Decher, Science 277 (1997) 1232.