Nanostructured and Spiky Gold Shell Growth on Magnetic Particles for SERS Applications.

Multifunctional micro- and nanoparticles have potential uses in advanced detection methods, such as the combined separation and detection of biomolecules. Combining multiple tasks is possible but requires the specific tailoring of these particles during synthesis or further functionalization. Here, we synthesized nanostructured gold shells on magnetic particle cores and demonstrated the use of them in surface-enhanced Raman scattering (SERS). To grow the gold shells, gold seeds were bound to silica-coated iron oxide aggregate particles. We explored different functional groups on the surface to achieve different interactions with gold seeds. Then, we used an aqueous cetyltrimethylammonium bromide (CTAB)-based strategy to grow the seeds into spikes. We investigated the influence of the surface chemistry on seed attachment and on further growth of spikes. We also explored different experimental conditions to achieve either spiky or bumpy plasmonic structures on the particles. We demonstrated that the particles showed SERS enhancement of a model Raman probe molecule, 2-mercaptopyrimidine, on the order of 104. We also investigated the impact of gold shell morphology-spiky or bumpy-on SERS enhancements and on particle stability over time. We found that spiky shells lead to greater enhancements, however their high aspect ratio structures are less stable and morphological changes occur more quickly than observed with bumpy shells.

Spiky gold shells on magnetic particles for DNA biosensors.

Combined separation and detection of biomolecules has the potential to speed up and improve the sensitivity of disease detection, environmental testing, and biomolecular analysis. In this work, we synthesized magnetic particles coated with spiky nanostructured gold shells and used them to magnetically separate out and detect oligonucleotides using SERS. The distance dependence of the SERS signal was then harnessed to detect DNA hybridization using a Raman label bound to a hairpin probe. The distance of the Raman label from the surface increased upon complementary DNA hybridization, leading to a decrease in signal intensity. This work demonstrates the use of the particles for combined separation and detection of oligonucleotides without the use of an extrinsic tag or secondary hybridization step.

Effect of SAM chain length and binding functions on protein adsorption: ß-lactoglobulin and apo-transferrin on gold.

Controlled immobilization of proteins is crucial in many applications, including biosensors. Allergen biosensing, for example, requires molecular recognition of suitably immobilized proteins by specific antibodies and sensitive measurement of this interaction. Self-assembled monolayers (SAMs), terminated by active functions, and are of great interest for the immobilization of biomolecules. The efficiency of further biorecognition involving molecules immobilized on these surfaces demonstrates an interesting dependence on the chain length and terminal function of the SAM. This motivated us to investigate adsorption of two proteins both known as milk allergens-ß-lactoglobulin and apo-transferrin-on amine-terminated SAMs. We varied the chain length by using either short or long chain amine-terminated thiols (cysteamine, CEA, and 11-mercaptoundecylamine, MUAM). We also investigated the influence of the addition of a rigid cross-linker, p-phenylene diisothiocyanate (PDITC), to these amine layers prior to protein adsorption. Protein binding was studied using polarization modulation-infrared reflection absorption spectroscopy (PM-IRRAS) and atomic force microscopy (AFM) to characterize their amount and dispersion. We found that protein immobilization varies with SAM chain length and is also influenced by the presence of a cross-linker. The presence of a rigid cross-linker favours the binding of proteins on long chain SAMs, while the effect is almost nonexistent on shorter chains. In addition, the presence of the cross-linker induces a better dispersion of the proteins on the surfaces, regardless of the length of the thiols forming the SAMs. The effects of chain length and chemistry of protein binding are discussed.

Surface plasmon resonance biosensors incorporating gold nanoparticles.

SPR biosensing is increasingly popular for the detection of a multitude of biomolecules. It offers label-free detection and study of proteins, nucleic acids, and other biomolecules in real time. A recent trend involves incorporation of AuNPs, either within the sensing surface itself or as signal enhancing tagging molecules. The importance of AuNP and detecting agent spacing is described and techniques using macromolecular spacing aids are highlighted. Recent methods to enhance SPR detection capabilities using gold nanoparticles are reviewed, as well as device fabrication and the results of incorporation. SPR detection is a highly versatile method for the detection of biomolecules and, with the incorporation of AuNPs, shows promise in extending it to a number of new applications.