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Prion Detection Through Organized Arrays of Gold Nanorods as SERS Substrates

Final Report Summary - PRIOSERS (Prion Detection Through Organized Arrays of Gold Nanorods as SERS Substrates)

The PRIOSERS project is a true reflection of the increasingly interdisciplinary character of modern science and lies at the frontier of scientific innovation. This proposal built on the genuinely original idea of developing rapid, inexpensive and sensitive methods for label-free analysis, based on nanoparticle organization and surface-enhanced Raman scattering spectroscopy (SERS) is intended to be used for the accurate and ultrasensitive fast detection of proteins, especially those related with pathogenic states such as prions (bovine spongiform encephalopathy, Creutzfeldt-Jakob disease but also related to Alzheimer and Parkinson diseases). As an intermediate goal, the project aimed the development of a highly effective plasmonic substrate based in the self-assembly of super-crystals with applicability for the ultrasensitive quantification of a variety of substance with importance in the fields of medicine, biology and environment. The PRIOSERS sensor is a collection of plasmonic nanoparticles organized at the microscale. Due to the close packing arrangement of the particles their plasmons will couple very strong forming localized areas with highly intense electromagnetic fields (hot spots) which will allow a SERS sensing capability at least up to nanomolar regime. To achieve this, Dr. Pazos-Perez pursues two different approaches to immobilize and organize the particles: Either on macroscopic substrates or, on microscale colloidal templates. The first part of the project involves the production of the individual building blocks: highly efficient and homogeneous plasmonic nanoparticles. Dr. Pazos Pazos Perez successfully developed new synthetic strategies to produce several types of novel plasmonic materials. He has synthesized extremely homogeneous gold nanorods with different aspect ratios (Fig. A, B, C),[1] highly homogeneous spiked nanoparticles composed of a penta-twinned core and five tips (Fig. D),[2] citrate stabilized gold and silver spherical nanoparticles with tuneable sizes,[3] and Silver nanorods epitaxially grown on gold (Fig. E, F, G).[4]

A second part of the project relay on the organization of plasmonic nanoparticles meanwhile studying their SERS efficiency.During this part of his IEF grant, Dr. Pazos-Perez successfully fabricated two different types of particle organizations: A) On macroscopic substrates producing supercrystals of gold nanorods with different sizes[1] (Fig. H, I) and also organizing them in periodic linear arrays (Fig.J K).[5] Moreover, he prepared as well periodic arrays of discrete supercrystals with various morphologies made of spherical gold nanoparticles (Fig. L-O).[6] B) On micron-size colloidal templates which consists of a collection of hybrid composite beads comprising a discrete colloidal stable polymer micro particle that supports plasmonic materials either at it surface[3] (Fig. P) or embedded on a porous polymer matrix (Fig. Q).[7] All different organizations were studied by SERS revealing that organizations of gold nanorods horizontally oriented to the substrate[1] generate side-to-side and tip-to-tip hotspots. However, the interaction between the gold surface and the analyte is hindered because of the presence of cetyltrimethylammonium bromide (CTAB) nevertheless, after a cleaning process with oxygen plasma, an increase in SERS intensity is observed showing detection limits in the nanomolar regime. For gold rods organize into parallel lines a reproducible tip-to-tip plasmonic coupling is generated with an intensification of the SERS signal with respect to a common commercial substrate of 40-fold.[5] On the other hand, when micro-structured assemblies of spherical Au nanoparticles on substrates[6] were produced, a consistent larger intensity than the one sustained by continuous films was obtained and, it was independent of the crystal geometry. Interestingly the absolute intensity increases as the side of the supercrystals increases. Moreover, besides substrates organizations, Dr. Pazos-Perez, has also investigated the SERS efficiency of the assemblies prepared on micron-size colloidal templates. A study on polymeric beads with Ag nanoparticles incorporated within its matrix[7] reveals that this system has approx. a nanomolar limit of detection, which is very similar to the one obtained for the organized NRs films. For the plasmonic particles assembled on the microbeads surface[3] it was observed that when incrementing the size of the individual nanoparticles the SERS intensities increase with particle size. However, when reaching certain sizes the radiation damping becomes dominant and significantly reduces the electromagnetic fields at the metal surface. For the las part of the project, Dr. Pazos-Perez decided to pursue a detection strategy for the identification and quantification of proteins in complex media. Specifically, he exploited the ability of the oncoprotein Myc to heterodimerize with its native protein partner Max, and therefore he designed a Max peptide receptor chemically modified to incorporate a thiophenol group (TP) at the N-terminal site. The TP functionality anchors the Max protein onto the metal substrate and works as an effective Raman spring to sense the structural rearrangements associated with the Myc/Max heterodimerization.[8]
In addition, Dr. Pazos-Perez decided also to use the acquired knowledge to design a detection strategy which instead of using organized particles for detection, is based on the particles self-assembly when the target analyte is present. To this end, he successfully used antibody functionalized plasmonic encoded nanoparticles to induce microscale assemblies of particles on the surface of target pathogens (i.e. bacteria) being capable of their multiplex detection in biological fluids.[9] Overall, despite that the final goal of the PRIOSERS project has not been reached, the successful achievement of most intermediate objectives has provide significant advances in several research fields. Dr. Pazos-Perez anticipates these findings of key importance in a number of different fields including diagnosis, biotechnology, agriculture, forensic science or homeland security. For this reason, a patent has been applied PCT/EP2015/067717.

References: [1] J. Phys. Chem. C. 2014, 118, 2809. [2] ACS Photonics 2014, 1, 1237. [3] Journal of Optics 2015, 17, 114012. [4] J. Phys. Chem. C. 2015, 119, 9513. [5] Part. Part. Syst. Charact. 2014, 31, 1134. [6] Nanoscale. 2016, DOI: 10.1039/c5nr09017b. [7] Journal of Colloid and Interface Science. 2015, 460, 128. [8] Manuscript n preparation 2016. [9] Scientific Reports, 2016, Just accepted.