Localized oblique-angle deposition: Ag nanorods on microstructured surfaces and their SERS characteristics

Microfluidic devices integrated with plasmonic nanostructures for sensitive fluorescent immunoassays

The robust identification and quantification of various biomarkers is of utmost significance in clinical diagnostics and precision medicine. Fluorescent immunoassays are widely used and considered as a gold standard for biomarker detection due to their high specificity and accuracy. However, current commercial immunoassay tests suffer from limited detection sensitivity and complicated, labor-intensive operation procedures, making them impractical for point-of-care diagnosis, particularly in resource-limited regions. Recently, microfluidic immunoassay devices integrated with plasmonic nanostructures have emerged as a powerful tool for sensitive detection of biomarkers, addressing specific issues, such as integration schemes, easy operation, multiplexed detection, and sensitivity enhancement. In this paper, we provide a discussion on the recent advances in the plasmonic nanostructures integrated with microfluidic devices for fluorescent immunoassays. We shed light on the nanofabrication strategies and various fluidic designs for rapid, sensitive, and highly efficient sensing of antigens. Finally, we share our perspectives on the potential directions of these integrated devices for practical applications.

Fabrication of porous ZnO/Co3O4 nanohybrids for the application of surface enhanced Raman scattering

With the growing use of various pesticides, it is important to develop facile and sensitive method to detect pesticides residues in food. Here, a semiconductor/magnetic hybrid material was used as surface enhanced Raman scattering (SERS) substrate to detect simulated residues. The representative sample of porous ZnO/Co₃O₄ nano-cube was fabricated by pyrolysis and calcination of Zn-Co ZIF, successively. The obtained hybrid of ZnO/Co₃O₄ was employed as substrate to detect of crystal violet (CV) and Rhodamine B (Rh B), and showed remarkable SERS performance. The detection limit of Rh B was 1 × 10^-10 M as well as CV of 1 × 10^-9 M. The results indicated that it was an ideal choice to improve the SERS property of transition metal oxide substrates by doping semiconductor. The semiconductor/magnetic hybrid material highlighted the obvious characteristics of low cost, facile preparation and ultra-low detection limit in the SERS measurements. The hybrids with the combination of semiconductor/magnetic properties showed a further widely application and development in SERS detection of pesticides residues.

Flexible SERS Substrate with a Ag-SiO2 Cosputtered Film for the Rapid and Convenient Detection of Thiram

It is very important to build uniform large-area dense hotspots to improve the surface-enhanced Raman scattering (SERS) detection limit. In our research, we designed and prepared a new flexibile SERS substrate with ultradense hot spots that has the advantages of high sensitivity, good repeatability, easy fabrication, and low cost. Due to the special dense hot spot structure, the substrate reaches a SERS enhancement factor of 2.1 × 10¹¹. Because of the excellent physical stability of polydimethylsiloxane, the substrate can be bent at will, and the SERS performance will not change with bending. This is very important to extract effective detection objects on complex surfaces. The substrate has good light transmittance and softness and can be directly attached to the detected agricultural products to realize real-time and rapid SERS monitoring. This structure exhibits extraordinary performance for thiram detection in the ultralow concentration range of 10^-13 M.

Monolithic integration of nanorod arrays on microfluidic chips for fast and sensitive one-step immunoassays

Here, we present integrated nanorod arrays on microfluidic chips for fast and sensitive flow-through immunoassays of physiologically relevant macromolecules. Dense arrays of Au nanorods are easily fabricated through one-step oblique angle deposition, which eliminates the requirement of advanced lithography methods. We report the utility of this plasmonic structure to improve the detection limit of the cardiac troponin I (cTnI) assay by over 6 × 105-fold, reaching down to 33.9 fg mL-1 (~1.4 fM), compared with an identical assay on glass substrates. Through monolithic integration with microfluidic elements, the device enables a flow-through assay for quantitative detection of cTnI in the serum with a detection sensitivity of 6.9 pg mL-1 (~0.3 pM) in <6 min, which was 4000 times lower than conventional glass devices. This ultrasensitive detection arises from the large surface area for antibody conjugation and metal-enhanced fluorescent signals through plasmonic nanostructures. Moreover, due to the parallel arrangement of flow paths, simultaneous detection of multiple cancer biomarkers, including prostate-specific antigen and carcinoembryonic antigen, has been fulfilled with increased signal-to-background ratios. Given the high performance of this assay, together with its simple fabrication process that is compatible with standard mass manufacturing techniques, we expect that the prepared integrated nanorod device can bring on-site point-of-care diagnosis closer to reality.

Morphological and SERS Properties of Silver Nanorod Array Films Fabricated by Oblique Thermal Evaporation at Various Substrate Temperatures

Aligned silver nanorod (AgNR) array films were fabricated by oblique thermal evaporation. The substrate temperature during evaporation was varied from 10 to 100 °C using a home-built water cooling system. Deposition angle and substrate temperature were found to be the most important parameters for the morphology of fabricated films. Especially, it was found that there exists a critical temperature at ~90 °C for the formation of the AgNR array. The highest enhancement factor of the surface-enhanced Raman scattering (SERS), observed in the Ag films coated with benzenethiol monolayer, was ~6 × 10(7). Hot spots, excited in narrow gaps between nanorods, were attributed to the huge enhancement factor by our finite-difference time-domain (FDTD) simulation reflecting the real morphology.

Co-Deposition of Gold Nanoparticles and Metalloporphyrin Using the Langmuir-Blodgett (LB) Technique for Surface-Enhanced Raman Scattering (SERS)

The synergistic effect produced by metallic nanoparticles when incorporated into different systems empowers a research field that is growing rapidly. In addition, organometallic materials are at the center of intensive research with diverse applications such as light-emitting devices, transistors, solar cells, and sensors. The Langmuir-Blodgett (LB) technique has proven to be suitable to address challenges inherent to organic devices, since the film properties can be tuned at the molecular level. Here we report a strategy to incorporate gold nanoparticles (AuNPs) into the LB film by co-deposition in order to achieve surface-enhanced Raman scattering (SERS) of the zinc(II)-protoporphyrin (IX) dimethyl ester (ZnPPIX-DME). Prior to the LB co-deposition, the properties of the Langmuir monolayer of ZnPPIX-DME at the air-water interface, containing AuNPs in the subphase, are studied through the surface-pressure versus mean molecular area (π-A) isotherms. The ZnPPIX-DME+AuNPs π-A isotherm presented a significant shift to higher molecular area, suggesting an interaction between both ZnPPIX-DME molecules and AuNPs. Those interactions are a key factor allowing the co-deposition of both AuNPs and ZnPPIX-DME molecules onto a solid substrate, thus forming the LB film. SERS of ZnPPIX-DME was successfully attained, ensuring the spatial distribution of the AuNPs. Higher enhancement factors were found at AuNP aggregates, as a result of the intense local electromagnetic field found in the metal nanoparticle aggregates. The main vibrational bands observed in the SERS spectra suggest a physical adsorption of the ZnPPIX-DME onto the surface of AuNPs. The latter is not only in agreement with the interactions pointed out by the π-A isotherms but also suggests that this interaction is kept upon LB film co-deposition.

Synthesis of surface-imprinted Ag nanoplates for detecting organic pollutants in water environments based on surface enhanced Raman scattering

Ag-molecularly imprinted polymer (MIP) hybrid composites (Ag@MIPs) were prepared for the ultra-sensitive detection of organic pollutants in water based on surface enhanced Raman scattering (SERS).