The collective oscillations of conduction band electrons which are in resonance with electromagnetic (EM) near-field stimulated by visible light in noble metal nanoparticles, known as localized surface plasmon resonance (LSPR), has attracted great interest in real-time label-free biosensing. Especially, LSPR nanostructures confine the plasmonic oscillation into the range comparable to the wavelength of light and greatly enhance the electric field near the nanoparticles. Owing to the locally enhanced EM field, it could provide high sensitivity to local refractive index variation. Thus, LSPR is an ideal candidate for real-time label-free biosensing applications.

Recently, extensive experimental and computational studies focused on the highly intensified electric fields known as “hot spots” and these “hot spots” were generated with configurations of narrow gaps, structures with small radius of curvature and interface between materials with different permittivity. These “hot spots” with enhanced electric field are believed to be highly sensitive to localized refractive index change or binding events. One of the strategies to improve the sensitivity of the LSPR sensor is to make full use of the “hot spots” in practical detections.

For LSPR biosensing, self-assembly gold nanoislands (SAM-AuNIs) have been established as a promising LSPR platform with strong plasmonic transition in the visible range. Meanwhile, it has the advantages of low material cost, simple fabrication, non-toxication and resistance to oxidation. Based on the SAM AuNIs structure, self-assembly silver nano-particles decorated on gold nano-islands (SAM Ag@AuNIs) as a sensitive sensing medium for LSPR biosensing applications was developed in this work. This was mainly through experimental investigations, and was also supported by computer simulations. The work aimed to establish that the SAM Ag@AuNIs LSPR sensor had excellent potential on label-free sensing applications. A direct functionalization scheme of AgNPs with biotin was also reported. The experimental and computational results indicated that the biotin molecule could selectively immobilize onto the AgNPs surface through the S-Ag bonds, resulting in site-specific functionalization. Meanwhile, the geometry of AgNPs/AuNIs “hot spots” in the vicinity of the AgNPs that helped to improve sensitivity. Combining with the biotinylated technology, the SAM Ag@AuNIs can serve as a desirable plasmonic medium in sensitive detection of biomolecules based on the exceptionally good limit of detection (LOD) of biotin at 11.87 pM.

Using the SAM Ag@AuNIs LSPR sensor and direct functionalization based on biotin, biotinylated antibody functionalized SAM Ag@AuNIs (BAF Ag@AuNIs) was established to study glioblastoma (GBM)’s formation and progression. GBM is a fatal and incurable brain tumour. Its late diagnosis at symptomatic stage via scanning using magnetic resonance imaging and computed tomography, and final determination by invasive biopsy is a significant hurdle to its therapy. The hypoxic GBM cells (GMs) increase the production and release of exosomes, which are 30-200 nm vesicles crossing the blood-brain-barrier, and enable exosomal biomarkers to be promising targets for the tracking of GBM malignancy. The SAM Ag@AuNIs LSPR biosensor sensitively detected CD63, an exosome marker, with sensitivity 1.71 times of that of AuNIs. Further, the BAF Ag@AuNIs biosensor sensitively detected enhanced level of MCT4,a crucial lactate transporter in cancer, in malignant hypoxic GMs-derived exosomes and GBM mouse blood–derived exosomes. Upregulation of MCT4 through metabolic reprogramming is one of the hallmarks in malignant GBM. Thus, the sensitive detection of enhanced level of GMs-derived-exosomal MCT4 in the blood of GBM mice by the BAF Ag@AuNIs biosensor could provide great insights into the development of a liquid biopsy method for early detection of GBM formation and progression in patients.

Titanium nitride (TiN) is another promising candidate for LSPR biosensing with properties including high hardness, outstanding corrosion resistance and excellent biocompatibility. However, it is hard to form nanoislands structure with self-assembly methods owning to its high melting temperature. Here, a TiN-Au hybrid nanoislands structure was reported to enhance the performance of plasmonic sensing within the visible range through the coupling effect. Through spectroscopic ellipsometry measurement, the optical constants and complex dielectric function were probed, and true optical constant data were obtained. It was found that TiN-Au hybrid nanoislands exhibited high extinction efficiency. When used in LSPR sensing of NaCl, the refractive index resolution was found to be 7.24×10^-8 RIU, which is comparable to that of SAM AuNIs (9.75×10^-8 RIU).

Therefore, by making use of “hot spots”, the proposed self-assembly LSPR sensors were found to possess enhanced plasmonic sensing performance, showing great potential for their applications in label-free real-time biosensing.