Gold-silver alloy film based surface plasmon resonance sensor for biomarker detection

In this study, we developed a gold‑silver alloy film based surface plasmon resonance (AuAg-SPR) sensor with wavelength interrogation to detect cancer antigen 125 (CA125) using a sandwich immunoassay. We first theoretically simulated the sensitivity of conventional gold film based SPR (Au-SPR) sensor and AuAg-SPR sensor, and conducted a series of experiments to investigate the sensitive characteristics of AuAg-SPR sensor, including the angle and refractive index (RI) sensitivity. We then conducted CA125 detection experiments on these two types of sensors. The results demonstrated that the limit of detection (LOD) of CA125 on the AuAg-SPR sensor was 0.1 U/mL (0.8 ng/mL) based on its direct reaction with an immobilised antibody, which was two orders of magnitude lower than that of the Au-SPR sensor (10 U/mL). The total changes in the resonance wavelength (∆λ_R) of the former were 1.7-fold those of the latter. The volume fractions of the adsorbates (f_ad) and effective RIs (n_adlayer) in each adlayer were then calculated and the effect of the antibody size on the detection results was analysed. The AuAg-SPR sensors had a higher sensitivity than the conventional Au-SPR sensors for detecting CA125 due to their electric field characteristics. Therefore, these will have better application prospects.

Optical Responses of Localized and Extended Modes in a Mesoporous Layer on Plasmonic Array to Isopropanol Vapor

Mesoporous silica features open and accessible pores that can intake substances from the outside. The combination of mesoporous silica with plasmonic nanostructures represents an interesting platform for an optical sensor based on the dependence of plasmonic modes on the refractive index of the medium in which metallic nanoparticles are embedded. However, so far only a limited number of plasmonic nanostructures are combined with mesoporous silica, including random dispersion of metallic nanoparticles and flat metallic thin films. In this study, we make a mesoporous silica layer on an aluminum nanocylinder array. Such plasmonic arrangements support both localized surface plasmon resonances (LSPRs) and extended modes which are the result of the hybridization of LSPRs and photonic modes extending into the mesoporous layer. We investigate in situ optical reflectance of this system under controlled pressure of isopropanol vapor. Upon exposure, the capillary condensation in the mesopores results in a gradual spectral shift of the reflectance. Our analysis demonstrates that such shifts depend largely on the nature of the modes; that is, the extended modes show larger shifts compared to localized ones. Our materials represent a useful platform for the field of environmental sensing.

Optimized Immobilization of Biomolecules on Nonspherical Gold Nanostructures for Efficient Localized Surface Plasmon Resonance Biosensing

Plasmonic biosensing techniques employ metal nanostructures, commonly gold (Au), often with biomolecules attached to their surfaces either directly or via other linkers. Various surface chemistry methods based on dispersion and covalent interactions are used to attach biomolecules to Au. As a result, when immobilizing a molecule on a metal surface, quantitative estimates of binding efficiency and stability of these surface chemistry methods are needed. Most prior work to compare such methods deals with bulk/thin film configurations or spherical nanoparticles, and very little is known about immobilization of biomolecules on plasmonic nanostructures of different shapes. Besides, due to rapid advancement of modern nanofabrication techniques, there is a growing need to determine an efficient surface chemistry method for immobilization of biomolecules on nonspherical plasmonic nanostructures. Previous comparison of immobilization methods on spherical Au nanoparticles has shown that physical adsorption resulted in the highest concentration of immobilized antibodies. In our work, we conducted a similar study and compared four representative Au surface functionalization methods as well as estimated how efficient these methods are at attaching biomolecules to nonspherical plasmonic Au nanostructures. We estimated the concentration of immobilized antibody that is specific to human C-reactive protein (anti-hCRP) by measuring the localized surface plasmon resonance (LSPR) shifts after exposing the surface of Au nanostructures to the antibody. Our results differ from the previously reported ones since the highest concentration of anti-hCRP was immobilized using 11-mercaptoundecanoic acid (MUA) chemistry. We demonstrated that immobilized antibodies retained their stability and specificity toward hCRP throughout the immunoassay when diluted hCRP or hCRP-spiked human serum samples were used. These findings have important implications for the fields of biosensing and diagnostics that employ nonspherical plasmonic nanostructures since an overall performance of these devices depends on efficient biomolecule immobilization.

Recent Advances in Spectroscopy Technology for Trace Analysis of Persistent Organic Pollutants

Persistent organic pollutants (POPs) have attracted significant attention because of their bioaccumulation, persistence, and toxicity. As anthropogenic products, POPs mainly contain polychlorinated biphenyls (PCBs), organochlorine pesticides (OPs), and polycyclic aromatic hydrocarbons (PAHs), and they pose a great threat to human health and the environment. To deal with these toxic contaminants, many different kinds of strategies for sensitively detecting POPs have been developed, such as high performance liquid chromatography (HPLC), surface enhanced Raman spectroscopy (SERS), and fluorescence. This paper mainly summarized the achievements of spectroscopy technologies, which generally consist of SERS, surface plasmon resonance (SPR), and fluorescence, in the detection of low-concentration POPs in different matrices. In addition, a retrospective summary is made on several critical considerations, such as sensitivity, specificity and reproducibility of these spectroscopy technologies in practical applications. Finally, some current challenges and future outlooks for these spectroscopy technologies are provided in regards to environmental analysis.

Hierarchical Thin Film Architectures for Enhanced Sensor Performance: Liquid Crystal-Mediated Electrochemical Synthesis of Nanostructured Imprinted Polymer Films for the Selective Recognition of Bupivacaine

Nanostructured bupivacaine-selective molecularly imprinted 3-aminophenylboronic acid-p-phenylenediamine co-polymer (MIP) films have been prepared on gold-coated quartz (Au/quartz) resonators by electrochemical synthesis under cyclic voltammetric conditions in a liquid crystalline (LC) medium (triton X-100/water). Films prepared in water and in the absence of template were used for control studies. Infrared spectroscopic studies demonstrated comparable chemical compositions for LC and control polymer films. SEM studies revealed that the topologies of the molecularly imprinted polymer films prepared in the LC medium (LC-MIP) exhibit discernible 40 nm thick nano-fiber structures, quite unlike the polymers prepared in the absence of the LC-phase. The sensitivity of the LC-MIP in a quartz crystal microbalance (QCM) sensor platform was 67.6 ± 4.9 Hz/mM under flow injection analysis (FIA) conditions, which was ≈250% higher than for the sensor prepared using the aqueous medium. Detection was possible at 100 nM (30 ng/mL), and discrimination of bupivacaine from closely related structural analogs was readily achieved as reflected in the corresponding stability constants of the MIP-analyte complexes. The facile fabrication and significant enhancement in sensor sensitivity together highlight the potential of this LC-based imprinting strategy for fabrication of polymeric materials with hierarchical architectures, in particular for use in surface-dependent application areas, e.g., biomaterials or sensing.