Immunoassay of Tumor Markers Based on Graphene Surface-Enhanced Raman Spectroscopy

Real-time monitoring of 2,5-dimethylpyrazine in solid, liquid, and gas phases during processing of fried skipjack tuna steaks using intestinal villus-like Ag nanoparticles@Au nanowires surface-enhanced Raman substrate chips

The production and distribution of 2,5-dimethylpyrazine (2,5-DMP), a key volatile flavor compound, are associated with the frying of skipjack tuna steaks. In this study, an intestinal villus-like chip was fabricated to quantitatively detect 2,5-DMP in multiphase systems based on surface-enhanced Raman spectroscopy (SERS). The chip exhibited excellent SERS performance with an enhancement factor of 1.16 × 108, excellent uniformity and reproducibility, and low detection limits of 6.49 pg/mL, 0.182, and 0.920 ng/mL for 2,5-DMP solid, liquid, and gas models, respectively. The results indicated the 2,5-DMP content with growth rates in the order of frying steam > frying steaks > frying oil, and the 2,5-DMP content in frying steam was linearly correlated (R2 = 0.992) with the trend of the acid value and total polar compounds in frying oil. Therefore, this strategy for achieving the reliable detection of 2,5-DMP can assist in monitoring the frying process and food safety.

A nano-imprinted graphene oxide-cellulose composite as a SERS active substrate

Cellulose is a sustainable material capable of forming optically active nanoarrays on its surface. We created a composite of cellulose acetate (CA) and graphene oxide (GO), by mixing GO (0.1 mg mL-1) into CA. This was then imprinted with nanoscale surface features that form Bragg-like modes in resonance with the excitation laser when a thin layer of silver is vapor deposited onto the surface of the substrate. The addition of GO leads to improved surface-enhanced Raman scattering (SERS) signal strengths, obtaining an average SERS signal increase of 1.4-fold following the inclusion of GO. The combination of photonic and electromagnetic effects with charge transfer-based processes that support the SERS chemical mechanism and the possible presence of electromagnetic hot spots from the roughened surface results in an enhanced SERS signal strength when GO is added. This work shows the potential for nanoimprinted graphene oxide/cellulose acetate composites as flexible sensor platforms to detect target molecules.

Synergistic enhancement mediated sensitive SERS-based immunoassay of PSA using versatile PDMS@AgNPs@ZIF-67 biomimetic substrates

Among various biomimetic polymer materials, polydimethylsiloxane (PDMS) stands out as an ideal matrix for surface-enhanced Raman scattering (SERS) due to its unique intrinsic Raman signal and tenacity. In order to realize the precise detection of prostate-specific antigen (PSA), we proposed a sandwich-type SERS-active immunostructure composed of PDMS@silver nanoparticles (Ag NPs)@ZIF-67 biomimetic film as the immunosubstrate and gold nanorods (Au NRs) as immunoprobes. Due to the synergistic effect of electromagnetic enhancement facilitated by biomimetic surfaces and chemical enhancement achieved by ZIF-67, this structure enabled an ultrasensitive and selective detection of PSA across a broad range from 10-3 to 10-9 mg/mL. The achieved limit of detection was as low as 3.0 × 10-10 mg/mL. Particularly, the intrinsic Raman signal of PDMS matrix at 2905 cm-1 was employed as a potential internal standard (IS) in the detection, achieving a high coefficient of determination (R2) value of 0.996. This multifunctional SERS substrate-mediated immunoassay holds vast potential for early diagnosis of prostate cancer, offering promising prospects for clinical applications.

Computational Insights into the Adsorption of Ligands on Gold Nanosurfaces

We study the adsorption process of model peptides, nucleobases, and selected standard ligands on gold through the development of a computational protocol based on fully atomistic classical molecular dynamics (MD) simulations combined with umbrella sampling techniques. The specific features of the interface components, namely, the molecule, the metallic substrate, and the solvent, are taken into account through different combinations of force fields (FFs), which are found to strongly affect the results, especially changing absolute and relative adsorption free energies and trends. Overall, noncovalent interactions drive the process along the adsorption pathways. Our findings also show that a suitable choice of the FF combinations can shed light on the affinity, position, orientation, and dynamic fluctuations of the target molecule with respect to the surface. The proposed protocol may help the understanding of the adsorption process at the microscopic level and may drive the in-silico design of biosensors for detection purposes.

Quantitative detection of purine from food products with different water activities using needle-based surface-enhanced Raman scattering sensors

Typically, for accurate quantitative tests of molecules, considering the actual solute concentration in the environment with different water activities (Aws) is essential. Accordingly, for effective detection of food substances, this paper proposes a non-destructive pluggable sensor to capture and monitor four free purines based on surface-enhanced Raman scattering characteristics such as sensitivity, uniformity, repeatability, and stability. In particular, we investigate the impact of Aw on the evaluation of purine detection and its deviation corrections. Furthermore, the recoveries of purine from three food products, including fish (Aw: 0.99), ham (Aw: 0.91), and bacon (Aw: 0.73), are subsequently explored to validate the reliability of the proposed method. The results indicate that the proposed non-destructive pluggable sensor performs better when the Aw is considered. Therefore, this strategy for achieving more reliable quantitative detection by rectifying deviations based on the Aw can significantly help monitor food quality.

Biomass-assisted fabrication of rGO-AuNPs as surface-enhanced Raman scattering substrates for in-situ monitoring methylene blue degradation

Reduced graphene oxide-gold nanoparticles nanocomposites (rGO-AuNPs) with high surface-enhanced Raman scattering (SERS) activity was created by biomass-assisted green synthesis with Lilium casa blanca petals biomass for the first time, and its application for methylene blue (MB) degradation was explored through in-situ monitoring. Lilium casa blanca petals biomass was used as a reducing agent to reduce GO and chloroauric acid successively when carrying out rGO-AuNPs in-situ synthesis while it also acted as a capping agent. The produced rGO had oxygen-containing functional groups which had an outstanding performance in enhancing the SERS effect. Characterization results confirmed that the AuNPs were grafted onto the rGO sheet, and the mechanism study showed that total flavonoids in Lilium casa blanca petals biomass were the main biological compounds involved in the reduction. rGO-AuNPs had a high Raman enhancement factor (EF) which could reach 3.88 × 10⁷. The synthesized nanocomposite also had a good catalytic activity that could be employed as catalyst in MB degradation, and it could complete degradation within 15min. The reaction rate increased linearly with the amount of rGO-AuNPs, and the degradation could be in-situ monitored both by UV and SERS.

Surface modification for improving immunoassay sensitivity

Immunoassays are widely performed in many fields such as biomarker discovery, proteomics, drug development, and clinical diagnosis. There is a growing need for high sensitivity of immunoassays to detect low abundance analytes. As a result, great effort has been made to improve the quality of surfaces, on which the immunoassay is performed. In this review article, we summarize the recent progress in surface modification strategies for improving the sensitivity of immunoassays. The surface modification strategies can be categorized into two groups: antifouling coatings to reduce background noise and nanostructured surfaces to amplify the signals. The first part of the review summarizes the common antifouling coating techniques to prevent nonspecific binding and reduce background noise. The techniques include hydrophilic polymer based self-assembled monomers, polymer brushes, and surface attached hydrogels, and omniphobicity based perfluorinated surfaces. In the second part, some common nanostructured surfaces to amplify the specific detection signals are introduced, including nanoparticle functionalized surfaces, two dimensional (2D) nanoarrays, and 2D nanomaterial coatings. The third part discusses the surface modification techniques for digital immunoassays. In the end, the challenges and the future perspectives of the surface modification techniques for immunoassays are presented.

Ultrasensitive detection of multiple cancer biomarkers by a triple cascade amplification strategy in combination with single particle inductively coupled plasma mass spectrometry

A versatile triple cascade amplification strategy was developed for ultrasensitive simultaneous detection of multiple cancer biomarkers using single particle inductively coupled plasma mass spectrometry (spICP-MS). The triple cascade amplification strategy consisted of an enhanced RecJ_f exonuclease-assisted target recycling amplification module, a hybridization chain reaction amplification module, and a signal amplification module based on DNA-templated multiple metal nanoclusters. In the enhanced RecJ_f exonuclease-assisted target recycling amplification module, the DNA bases at the 5' ends of aptamers for specific recognition of biomarkers were deliberately replaced by the corresponding RNA bases to enhance amplification efficiency. The signal amplification module based on DNA-templated multiple metal nanoclusters was innovatively used to amplify the signals measured by spICP-MS and at the same time effectively suppress possible background interferences. The proposed spICP-MS platform achieved satisfactory quantitative results for both carcinoembryonic antigen (CEA) and a-fetoprotein (AFP) in human serum samples with accuracy comparable to that of the commercial ELISA kits. Moreover, it has wide dynamic ranges for both CEA (0.01-100 ng/mL) and AFP (0.01-200 ng/mL). The limit of detection for CEA and AFP was 0.6 and 0.5 pg/mL, respectively. Compared with conventional biomarkers detection methods, the proposed spICP-MS platform has the advantages of operational simplicity, ultra-high sensitivity, wide dynamic range, and low background. Therefore, it is reasonable to expect that the proposed spICP-MS platform can be further developed to be a promising alternative tool for biomarker detection in fields of clinical diagnosis and biomedical research.

Photochromic immunoassay for tumor marker detection based on ZnO/AgI nanophotocatalyst

A photochromic immunoassay was built for tumor marker detection based on ZnO/AgI nanophotocatalyst. Frist, ZnO/AgI nanoparticles were synthesized and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), and Fourier transform infrared spectrometry (FTIR). The color development is caused by tetramethyl benzidine (TMB) solution oxidated by ZnO/AgI nanomaterials. The electron transitions in ZnO/AgI nanomaterials are driven by visible light irradiation, generating photogenerated hole and oxidizing TMB to blue solution. Appropriate band width between ZnO and AgI promotes separation of photogenerated electrons and holes and enhances oxidation efficiency. A sandwich-type immunoassay was constructed based on ZnO/AgI nanomaterial as labels. The absorbance at 650 nm of reaction solution is positively correlated with antigen concentration. The developed immunoassay showed good performance for carcinoma embryonic antigen (CEA) detection in the range 0.1-7.0 ng/mL with a detection limit of 65 pg/mL. The photochromic immunoassay also exhibited preferable selectivity, repeatability, and stability. A novel colorimetric immunoassay was constructed based on ZnO/AgI photocatalyst. ZnO/AgI nanomaterials occur electron transitions under visible light irradiation and generate photogenerated hole, which can oxidize TMB to blue solution. Carcinoembryonic antigen in sample was detected sensitively due to the high catalytic efficiency of ZnO/AgI nanomaterials.

Nonmetallic SERS-based immunosensor byintegrating MoS2 nanoflower and nanosheet towards the direct serum detection of carbohydrate antigen 19-9

Recently, nonmetallic substrates have stimulated great interest in surface-enhanced Raman scattering (SERS)-based immunoassay owing to their good uniformity, stability, and biocompatibility. In this context, a novel nonmetallic SERS-based immunoassay mediated by two-dimensional molybdenum disulfide (MoS₂) was delivered for the sensitive and specific monitoring of carbohydrate antigen 19-9 (CA19-9). The effective enrichment of molecules on the large active surfaces of MoS₂ as well as potential 532-nm laser-induced charge transfer resonances between them engendered desirable enhancement factor values at the level of 10⁵. Intriguingly, a sandwich immunocomplex combined MoS₂ nanoflower and nanosheet exhibited not only a wide linear range from 5 × 10^-4 to 1 × 10² IU·mL^-1 but also a limit of detection as low as 3.43 × 10^-4 IU·mL^-1 towards CA19-9. More meaningful, the analytical result for clinical patient serum sample was basically compared with the conventional chemiluminescent immunoassay. Such a novel nonmetallic SERS-based immunosensor with desirable biocompatibility and sensitivity is promising for clinical diagnosis.

A multimer-based SERS aptasensor for highly sensitive and homogeneous assay of carcinoembryonic antigens

Carcinoembryonic antigens (CEAs) are known as one of the most common tumor markers. Their facile and affordable detection is critical for early diagnosis of malignant tumors, especially in resource-constrained settings. Here, we report a novel multimer-based surface-enhanced Raman scattering (SERS) aptasensor for a specific CEA assay. The aptasensor is fabricated through aptamer-assisted self-assembly of silver-coated gold nanoparticles (Au@Ag NPs), and the self-assembled multimeric structure possesses abundant hot-spots to provide high SERS response. When CEA is introduced, the specific recognition of CEA by aptamers will lead to the disassembly of Au@Ag multimers due to the lack of a bridging aptamer between Au@Ag NPs. As a result, the number of hot-spots in the multimeric system is decreased, and the intensity at 1585 cm-1 of the SERS reporter (4-mercaptobenzoic acid, 4-MBA) on the surface of NPs will also be decreased. The Raman intensity is proportional to the logarithm of the concentration of CEA. The detection sensitivity can be down to the pg mL-1 level. The analytical method only needs a droplet of 2 μL of sample, and the detection time is less than 20 min. The multimer-based SERS aptasensor can be applied in sensitive and inexpensive detection of CEA in serum samples.