RNA aptamer-based optical nanostructured sensor for highly sensitive and label-free detection of antigen–antibody reactions

A number of modern techniques to study the optical properties of metallic nanostructures such as localized surface plasmon resonance, interference, and surface enhanced Raman scattering were carefully studied recently[1-3]. In the field of analytical chemistry, the absorbance or scattering spectra of nanostructured sensors based on localized surface plasmon resonance receive more attention because of their high sensitivity and reproducibility [4-6].Depending on material types, nanostructure sizes and shapes, the absorbance spectrum of nanostructure possesses various levels of sensitivity in accordance to changes in interfacial refractive index (RI) of surrounding media and biomolecular bindings which occur on the nanostructure’s surface [7-10]. These modifications on the nanostructure surface cause corresponding changes of peak intensity or wavelength shift of absorbance spectra that are efficiently detectable signals in nanostructured sensors [11, 12].Glass substrate with immobilized gold nanoparticles is one of the most fascinating candidates for biological applications [13-15]. Nath and Chilkoti immobilized gold nanoparticles on the glass substrate and showed successful attempts of their substrates to study the streptavidin–biotin interactions using conventional transmission spectropho- tometry [14, 15]. Similarly, K. Fujiwara et al. fixed human serum albumin (HSA) on a gold nanoparticle surface and examined different bindings of anti-HSA antibody HSA with a detection limit set to 10 nM of anti-HAS antibodies [16]. However, several drawbacks of these goldnanoparticles-based plasmonic sensors have been pointed out. One of them is the repulsion forces from negative charge of citrate ions on gold nanoparticles surface. It prevents immobilized gold nanoparticles from gathering with high density, thus reduce the biosensing capacity of these biosensors. The second reason is a requirement of complicated chemistry required to form a self-assembled monolayer (SAM) of gold nanoparticles. Consequently, the slightest defect in the uniformity of SAM might cause significant problems in results reproducibility and reliability. Besides, integrating this plasmonic substrate into other technologies as well as their miniaturizations in a compact system is limited because of the necessity of Kretschmann configuration in its total internal reflection mode.In order to surmount these issues and improve the density of immobilized metallic nanoparticles, we developed a gold-capped nanoparticles substrate and achieved a highly sensitive and label-free detection of the antigenantibody reactions [17, 18], DNA–DNA hybridization [19],peptides–membrane [20], and cell metabolites–receptors interactions [21]. The proposed device also performed multi-detection of clinically important proteins by spotting different antibodies on the device’s surface [22]. In its construction, the silica nanoparticles which were immobilized with high density on the surface, played the role as the “core”, and thin gold films were used as the “cap” coated on the top of that “core”. The device was simple to implement, requiring only a UVVis spectrophotometer ora flatbed scanner, and was suitable for multiplex analysis with highthroughput monitoring in array-based format.In this work, the sensing capacity of gold-capped nanoparticle substrates was examined in detail throughout the visible region in various refractive index solutions (from ethanol to toluene). Based on our gold-capped nanoparticle substrate, we presented a novel optical protein nanostructured sensor for label-free detection of specific target molecules with high sensitivity and selectivity. The main work focused on immobilizing a thiolated RNA aptamer which is able to catch Fc region of antibody, thus promising an ability to orient the antibody on the sensor’s surface. Several truncated and modified aptamers were designed and the optimized aptamer which had the highest affinity to the Fc portion of human IgG1 subclass was selected using surface plasmon resonance [23]. We could detect various fibrinogen concentrations when anti-fibrinogen antibodies were immobilized on the gold-capped nanoparticles sur face. Consequently, we could perform detections onantigen–antibody interactions rapidly (less than 1.5 h for all steps) in a user-friendly method, and it served as an alternative for conventional techniques [17, 22]. This excellent approach was a good demonstration for our gold-capped nanoparticles substrate which had high sensitivity and selectivity, as well as its possibility of a wide range of analytical calibration.

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