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RESEARCH PROFILES Putting long-range surface plasmons to work
probes. A self-assembled monolayer of an amine-terminated alkanethiol was first deposited on the gold. A heterobifunctional linker called SSMC was then spotRobert Corn at the University of Califorted onto the monolayer. The linker pernia, Irvine, says that his group is “conmitted 5′-thiol-modified ssDNA probes stantly trying to improve SPR [surface to attach to the surface. plasmon resonance] imaging sensitivity.” For the analysis, a solution of target The investigators previously demonstratssDNA molecules that could only hyed an enzymatic amplification method bridize with one type of ssDNA that increased the sensitivity of SPR 100 probe was incubated on the imaging by 106 (Anal. Chem. array. Corn and colleagues 2004, 76, 6173–6178; 386 A). used LRSPs to image a 700In this issue of Analytical Chem80 µm area on the array, both beistry (pp 3904–3907), they report fore and after incubation with on the use of long-range surface 60 the target DNA molecules. plasmons (LRSPs) that can im1.2° 0.15° The differences in LRSP reflecprove the sensitivity of SPR imagtivity produced a qualitative ing by 20%. 40 Glass image of the array, displaying The concept of LRSPs was Glass 1180 nm Cytop the hybridized and unhybridfirst predicted and demonstrated 20 45 nm Au ized DNA molecules. in the early 1980s. LRSPs are 32 nm Au Water Water The investigators repeated surface-bound electromagnetic 0 the experiment more quantitawaves that have longer propaga54 56 46 52 48 50 tively with a series of different tion lengths and narrower anguAngle (degrees) concentrations of the target lar resonance curves than conDNA solution. They found ventional surface plasmons. But The resonance curve from a multilayer chip that produces that the LRSPs allowed an indifficulties in generating the LRSPs (red) is sharper and more symmetric compared with that crease of 20% in measurement waves of LRSPs stalled potential from a conventional SPR chip (blue). The polymer Cytop in the sensitivity compared with a applications. multilayer chip is critical for the production of LRSPs. similar experiment carried out Corn and colleagues have now on a conventional SPR chip. Corn and colleagues carried out both developed a method that allows LRSPs to Theoretical predictions suggest that a Fresnel calculations and scanning-angle be easily produced. The key to creating 40% improvement in LRSP-based imagLRSPs is to sandwich a noble-metal layer, SPR measurements to demonstrate the typically gold, between two dielectric ma- creation of LRSPs on the multilayer chip. ing is possible. The investigators say that the system can be further optimized by, They found the resonance curve for the terials of closely matching indices of refor example, ensuring that the incident fraction. For biological applications, water Cytop chip to be sharper and more symwhite-light beam is properly collimated metric compared with that for a convenusually is one of the materials. “The hard and doesn’t have much angular spread. tional SPR chip without the polymer. thing is finding a material that has the Corn also suggests that wavelengths The investigators analyzed the theoindex of refraction of water, is robust, and closer to the IR region could be used to retical dependence of the LRSP resocan be stuck underneath the gold,” exnance curve on the thickness of gold for generate LRSPs with longer propagation plains Corn. “My postdoc tested 10 diflengths. ferent inorganic and organic materials be- a Cytop layer of 1180 nm. Their analyCorn and colleagues are continuing sis showed that a 30-nm thickness was fore finding one that worked.” their quest to improve the measureThe investigators eventually found an optimal because the intensity of the ment sensitivity of SPR imaging, but plasmon electric field was the highest. amorphous fluoropolymer called Cytop they are also focusing on applications Next, the investigators demonstrated (refractive index = 1.34) that met their for LRSPs. “Propagation lengths of criteria. They built a multilayer chip that for the first time that LRSPs could be SPR are ~10 µm, [which is] not quite consisted of glass spin-coated with 1180 used to monitor biomolecular interacbig enough to easily make devices. One nm of Cytop. The Cytop was next coat- tions. In particular, they measured the thing we’re thinking about is making interactions between molecules of sined with a 32-nm layer of gold by vapor optoelectronic devices with LRSP cougle-stranded DNA (ssDNA). deposition. A 1-nm layer of chromium plers,” says Corn. a An array was made on an LRSP chip had to be laid down first to help the —Rajendrani Mukhopadhyay by attaching two different 16-mer ssDNA gold adhere to the polymer. ROBERT CORN
% Reflectivity
When an aqueous solution covered the chip, the gold layer, now embedded between two symmetric dielectric materials, supported the production of LRSPs. The LRSPs propagated ~200 µm for an incident wavelength of 814 nm. Conventional surface plasmons propagate 10–20 µm for the same wavelength of incident light.
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