Tracking chemical interactions of folic acid on gold surface by SERS spectroscopy

SERS analysis, DFT, and solution effects regarding the structural and optical characteristics of folic acid biomolecule adsorbed on a Cu3 metal cluster

The optical characteristics of folic acid (ABP) and metal clusters of copper (Cu3) at various locations were investigated by means of density functional theory (DFT) computations. Mulliken charge analysis and molecular electrostatic potential (MEP) surface show how charge moves from Cu3 to ABP through the various groups. The peak in the UV-Vis spectra of ABP-Cu3 is caused by bonding and anti-bonding orbitals. In both vacuum and aqueous conditions, the polarizability values of ABP-Cu3 cluster are significantly higher than those of pure ABP, indicating a possible enhancement of the nonlinear optical (NLO) effect. Our research investigates the possibility of using ABP adsorbed metal clusters for NLO materials. Surface enhanced Raman scattering (SERS) in the ABP adsorbed metal clusters enhances the vibrational modes of ABP. Adsorption energies are found to be in the range -17.08 to -58.52 kcal/mol in vacuum and -53.34 to -93.44 kcal/mol in aqueous medium for the different configurations for ABP-Cu3. It indicates that metal clusters adsorbed by ABP are stable in the aqueous media. Experimental IR and UV-Vis of ABP is in agreement with theoretically predicted ones.

Surface-functionalized SERS platform for deep learning-assisted diagnosis of Alzheimer's disease

Early diagnosis of Alzheimer's disease is crucial to stall the deterioration of brain function, but conventional diagnostic methods require complicated analytical procedures or inflict acute pain on the patient. Then, label-free Surface-enhanced Raman spectroscopy (SERS) analysis of blood-based biomarkers is a convenient alternative to rapidly obtain spectral information from biofluids. However, despite the rapid acquisition of spectral information from biofluids, it is challenging to distinguish spectral features of biomarkers due to interference from biofluidic components. Here, we introduce a deep learning-assisted, SERS-based platform for separate analysis of blood-based amyloid β (1-42) and metabolites, enabling the diagnosis of Alzheimer's disease. SERS substrates consisting of Au nanowire arrays are fabricated and functionalized in two characteristic ways to compare the validity of different Alzheimer's disease biomarkers measured on our SERS system. The 6E10 antibody is immobilized for the capture of amyloid β (1-42) and analysis of its oligomerization process, while various self-assembled monolayers are attached for different dipole interactions with blood-based metabolites. Ultimately, SERS spectra of blood plasma of Alzheimer's disease patients and human controls are measured on the substrates and classified via advanced deep learning techniques that automatically extract informative features to learn generalizable representations. Accuracies up to 99.5% are achieved for metabolite-based analyses, which are verified with an explainable artificial intelligence technique that identifies key spectral features used for classification and for deducing significant biomarkers.

Ultra-clean ternary Au/Ag/AgCl nanoclusters favoring cryogenic temperature-boosted broadband SERS ultrasensitive detection

Exploring multifunctional surface-enhanced Raman scattering (SERS) substrates with high sensitivity, broadband response property and reliable practicability should be required for ultrasensitive molecular detection in complex environments, which is heavily dependent on the photo-induced charge transfer (PICT) efficiency realized on the desirable nano-architectures. Herein, we introduce ultra-clean ternary Au/Ag/AgCl nanoclusters (NCs) with broadband resonance crossing the visible light to near-infrared region created by one step laser irradiation of mixed metal ion solution. Interestingly, the surface defects and interaction among these unique cluster-like ternary nanostructures would be further enhanced by thermal annealing treatment at 300°C, providing higher broadband SERS activities than the reference ternary nanoparticles under 457, 532, 633, 785, and 1064 nm wavelengths excitation. More importantly, the further promoted SERS activities of the resultant Au/Ag/AgCl NCs with achievable ∼5-fold enhancement than the initial one can be conventionally realized by simplistically declining the temperature from normal 20°C to cryogenic condition at about -196°C, due to the lower temperature-suppressed non-radiative recombination of lattice thermal phonons and photogenerated electrons. The cryogenic temperature-boosted SERS of the resultant Au/Ag/AgCl NCs enables the limit of detection (LOD) of folic acid (FA) biomolecules to be achieved as low as 10-12 M, which is obviously better than that of 10-9 M at room temperature condition. Overall, the smart Au/Ag/AgCl NCs-based broadband SERS sensor provides a new avenue for ultrasensitive biomolecular monitoring at cryogenic condition.

Can DFT Calculations Provide Useful Information for SERS Applications?

Density functional theory (DFT) calculations allow us to reproduce the SERS (surface-enhanced Raman scattering) spectra of molecules adsorbed on nanostructured metal surfaces and extract the most information this spectroscopy is potentially able to provide. The latter point mainly concerns the anchoring mechanism and the bond strength between molecule and metal as well as the structural and electronic modifications of the adsorbed molecule. These findings are of fundamental importance for the application of this spectroscopic technique. This review presents and discusses some SERS-DFT studies carried out in Italy as a collaboration between the universities of Modena and Reggio-Emilia and of Florence, giving an overview of the information that we can extract with a combination of experimental SERS spectra and DFT modeling. In addition, a selection of the most recent studies and advancements on the DFT approach to SERS spectroscopy is reported with commentary.

Surface enhanced fluorescence immuno-biosensor based on gold nanorods

Gold nanoparticles (AuNPs) are attractive structures for biosensing, most due to different properties at nanoscale and biocompatibility. Localized surface plasmon resonance (LSPR) is one of these properties; LSPR enable the electromagnetic field enhancement closer to metallic surface, which allows surface-enhanced spectroscopies, like surface enhanced fluorescence (SEF). In this study, an immuno-biosensor based on gold nanorods (AuNRs) and SEF was constructed for simple and fast analysis to detect albumin antibody (anti-BSA) using antigen-antibody (anti-BSA/BSA) interaction as the biorecognition model. AuNRs were presented in two distinct configurations, in suspension (S-AuNRs) and adsorbed on glass slides (AuNRs-chip), and the detection was performed through an extrinsic method, wherein the SEF signal of a reporter molecule (IR-820 cyanine-type dye) was monitored. The analyte detection was evidenced by SEF mapping, where the average signal in the presence of anti-BSA was three times more intense than for the assay in the absence of analyte. A digital protocol was proposed to simplify the spectroscopic data analysis and reduce the intensity variability; in this protocol the number of positive events in the presence of anti-BSA is much larger (around two times) compared to the absence of analyte. The AuNRs based SEF immuno-biosensor allowed an efficient and simple analysis with specific biorecognition and may contribute as an efficient spectroscopy platform for immuno-biosensing.

Folic acid and sodium folate salts: Thermal behavior and spectroscopic (IR, Raman, and solid-state 13C NMR) characterization

Folic acid (FA; vitamin B9) and its associated sodium salts, strongly relevant for many scientific and technological applications - from nutrition to pharmacology and nanomedicine, suffer from a lack of characterization combining experimental and theoretical. In this work, a spectroscopic investigation of FA and its synthesized sodium salts in the form of dianion (Na₂HFol) or trianion (Na₃Fol) was scrutinized in their solid state. The spectroscopic (infrared, Raman, and solid state ¹³C-nuclear magnetic resonance) data interpretation was supported by theoretical calculations using the Density Functional Theory (DFT). Additionally, the compounds were characterized by UV-VIS diffuse-reflectance spectroscopy, combined thermal analysis (TG/DTG-DSC) coupled to mass spectrometry, and X-ray diffractometry. The main signatures of each species were identified, as well as the influence of the protonation level on their physicochemical properties. These distinct properties for the three compounds are mainly based on signals assigned to glutamic acid (glutamate) and pterin (neutral or anionic) moieties. This work should help developing new products based on FA or its anionic forms, such as theragnostic/drug delivery systems, supramolecular structures, nanocarbons, or metal complexes.

Accessing BCG in infected macrophages by antibody-mediated drug delivery system and tracking by surface-enhanced Raman scattering spectroscopy

Gold nanoparticles (AuNP) modified with antibody and rifampicin (RP) were tested against Mycobacterium bovis Bacillus Calmette-Guérin (BCG), which previously generated in vitro infection of macrophages from mice. Such a drug delivery system works as nanocarrier for RP and presented lower toxicity for macrophages cells than each separated component. Surface-enhanced Raman scattering (SERS) spectroscopy and fluorescence microscopy were used as analytical tools for the characterization of the internalization of gold nanocarriers into macrophage cells. The effective antibiotic action of RP, when combined with gold nanocarrier, was confirmed by dead-live assay of BCG bacteria lysed from macrophages after incubation. Such results indicate the delivery of RP to BCG bacteria, which were infecting macrophages, occurred with remarkable efficiency. It was rationalized based on the strategy used for the adsorption of antibody molecules on gold surface.

Highly Sensitive Electrochemical Biosensor Using Folic Acid-Modified Reduced Graphene Oxide for the Detection of Cancer Biomarker

The detection of cancer biomarkers in the early stages could prevent cancer-related deaths significantly. Nanomaterials combined with biomolecules are extensively used in drug delivery, imaging, and sensing applications by targeting the overexpressed cancer proteins such as folate receptors (FRs) to control the disease by providing earlier treatments. In this investigation, biocompatible reduced graphene oxide (rGO) nanosheets combined with folic acid (FA)-a vitamin with high bioaffinity to FRs-is utilized to develop an electrochemical sensor for cancer detection. To mimic the cancer cell environment, FR-β protein is used to evaluate the response of the rGO-FA sensor. The formation of the rGO-FA nanocomposite was confirmed through various characterization techniques. A glassy carbon (GC) electrode was then modified with the obtained rGO-FA and analyzed via differential pulse voltammetry (DPV) for its specific detection towards FRs. Using the DPV technique, the rGO-FA-modified electrode exhibited a limit of detection (LOD) of 1.69 pM, determined in a linear concentration range from 6 to 100 pM. This excellent electrochemical performance towards FRs detection could provide a significant contribution towards future cancer diagnosis. Moreover, the rGO-FA sensing platform also showed excellent specificity and reliability when tested against similar interfering biomolecules. This rGO-FA sensor offers a great promise to the future medical industry through its highly sensitive detection towards FRs in a fast, reliable, and economical way.