Large-Scale Fabrication of Nanostructure on Bio-Metallic Substrate for Surface Enhanced Raman and Fluorescence Scattering

Cancer Marker Immunosensing through Surface-Enhanced Photoluminescence on Nanostructured Silver Substrates

Nanostructured noble metal surfaces enhance the photoluminescence emitted by fluorescent molecules, permitting the development of highly sensitive fluorescence immunoassays. To this end, surfaces with silicon nanowires decorated with silver nanoparticles in the form of dendrites or aggregates were evaluated as substrates for the immunochemical detection of two ovarian cancer indicators, carbohydrate antigen 125 (CA125) and human epididymis protein 4 (HE4). The substrates were prepared by metal-enhanced chemical etching of silicon wafers to create, in one step, silicon nanowires and silver nanoparticles on top of them. For both analytes, non-competitive immunoassays were developed using pairs of highly specific monoclonal antibodies, one for analyte capture on the substrate and the other for detection. In order to facilitate the identification of the immunocomplexes through a reaction with streptavidin labeled with Rhodamine Red-X, the detection antibodies were biotinylated. An in-house-developed optical set-up was used for photoluminescence signal measurements after assay completion. The detection limits achieved were 2.5 U/mL and 3.12 pM for CA125 and HE4, respectively, with linear dynamic ranges extending up to 500 U/mL for CA125 and up to 500 pM for HE4, covering the concentration ranges of both healthy and ovarian cancer patients. Thus, the proposed method could be implemented for the early diagnosis and/or prognosis and monitoring of ovarian cancer.

Self-assembled PVP-gold nanostar films as plasmonic substrates for surface-enhanced spectroscopies: influence of the polymeric coating on the enhancement efficiency

Colloidal nanoparticles exhibiting anisotropic morphologies are preferred in the structural design of spectroscopically active substrates due to the remarkable optical properties of this type of nano-object. In the particular case of star-like nanoparticles, their sharp tips can act as antennae for capturing and amplifying the incident light, as well as for enhancing the light emitted by nearby fluorophores or the scattering efficiency of Raman active molecules. In the current work, we aimed to implement such star-shaped nanoparticles in the fabrication of nanoparticle films and explore their use as solid plasmonic substrates for surface-enhanced optical spectroscopies. High-density, compact and robust self-assembled gold nanostar films were prepared by directly depositing them from aqueous colloidal suspension on polystyrene plates through convective self-assembly. We investigated the role of the polymeric coating, herein polyvinylpyrrolidone (PVP), in the particle assembly process, the resulting morphology and consequently, the plasmonic response of the obtained films. The efficacy of the plasmonic films as dual-mode surface-enhanced fluorescence (SEF) and surface-enhanced Raman scattering (SERS) substrates was evidenced by testing Nile Blue A (NB) and Rhodamine 800 (Rh800) molecular chromophores under visible (633 nm) versus NIR (785 nm) laser excitation. Steady-state and time-resolved fluorescence investigations highlight the fluorescence intensity and fluorescence lifetime modification effects. The experimental results were corroborated with theoretical modelling by finite-difference time-domain (FDTD) simulations. Furthermore, to prove the extended applicability of the proposed substrates in the detection of biologically relevant molecules, we tested their SERS efficiency for sensing metanephrine, a metabolite currently used for the biochemical diagnosis of neuroendocrine tumors, at concentration levels similar to other catecholamine metabolites.

Rapid identification and quantification of diquat in biological fluids within 30 s using a portable Raman spectrometer

Rapid detection of diquat (DQ) is essential in clinical diagnosis and rescue. Here, we developed a fast, simple-yet-practical detection strategy for the reliable identification and quantification of DQ in biological fluids. Based on surface-enhanced Raman spectroscopy (SERS), point-of-care detection was realized under the acidic condition with gold nanoparticles as the substrate. Under optimal experimental conditions, the detection limits of the strategy were 17.5 ppb and 1.99 ppm in human urine and gastric juice, respectively. High specificity and selectivity of the SERS strategy were demonstrated using common pesticides and coexisting biological substances. The method was also used to detect biofluids from 5 patients and urine samples from 10 healthy volunteers. The results were in high agreement with spectrophotometric and clinical liquid chromatography-mass spectrometry methods. The volume of urine samples required for this technique is merely 20 μL, and no preparation of the samples is required. Compared to traditional methods used in clinical settings, SERS-based methods are capable of real-time measurements that accurately provide rapid detection and response in non-laboratory settings, with great potential for on-site and point-of-care testing.

Multifunctional Au@Ag@SiO2 Core-Shell-Shell Nanoparticles for Metal-Enhanced Fluorescence, Surface-Enhanced Raman Scattering, and Photocatalysis Applications

Au@Ag@SiO₂ core-shell-shell nanoparticles (NPs) were prepared by a facile one-pot synthetic technique. The Au@Ag core size and SiO₂ shell thicknesses are readily controlled by adjusting the precursor concentration. The multilayered NPs with dielectric SiO₂ outer shells and bimetallic Au@Ag cores exhibited both the chemical stability of Au with the high scattering efficiency of Ag. Furthermore, the SiO₂ shell is beneficial to the metal-enhanced fluorescence for biomedical applications. Metal-enhanced fluorescence, surface-enhanced Raman scattering, and photocatalytic activities of silica-coated Au@Ag, Ag, Au, and Au/Ag core-shell NPs were compared and discussed. The size and structure of Au@Ag@SiO₂ core-shell-shell NPs were optimized to maximize their optical and catalytic activities.

Rapid Quantitative Detection of Voriconazole in Human Plasma Using Surface-Enhanced Raman Scattering

There is an increasing demand for rapid detection techniques for monitoring the therapeutic concentration of voriconazole (VRC) in human biological fluids. Herein, a rapid and selective surface-enhanced Raman scatting method for point-of-care determination of VRC in human plasma was developed via a portable Raman spectrometer. This approach has enabled the quantification of the VRC spiked into human plasma at clinical relevant concentrations. A gold nanoparticle solution (Au sol) was used as the SERS substrate, and the agglomerating conditions on its sensitivity were optimized. The method involves the formation of hot spots, and the signal of VRC molecules adsorbed on the surface of the SERS hot spot was amplified by 10⁵. The calibration curve was linear in the range of 0.02-10 ppm, with satisfactory repeatability. The limit of detection was as low as 12.3 ppb. The variation in VRC spectra over time on different substrates demonstrated good reproducibility. Notably, the salting-out extraction method developed in this study was rapid and suitable for the quantitation of drugs in biological samples. Compared with traditional methods, this approach allows for the point-of-care quantification of VRC directly in a complex matrix, which may open up new exciting opportunities for future use of the SERS technique in clinical applications.

Classification of Preeclamptic Placental Extracellular Vesicles Using Femtosecond Laser Fabricated Nanoplasmonic Sensors

Placental extracellular vesicles (EVs) play an essential role in pregnancy by protecting and transporting diverse biomolecules that aid in fetomaternal communication. However, in preeclampsia, they have also been implicated in contributing to disease progression. Despite their potential clinical value, current technologies cannot provide a rapid and effective means of differentiating between healthy and diseased placental EVs. To address this, a fabrication process called laser-induced nanostructuring of SERS-active thin films (LINST) was developed to produce scalable nanoplasmonic substrates that provide exceptional Raman signal enhancement and allow the biochemical fingerprinting of EVs. After validating the performance of LINST substrates with chemical standards, placental EVs from tissue explant cultures were characterized, demonstrating that preeclamptic and normotensive placental EVs have classifiably distinct Raman spectra following the application of advanced machine learning algorithms. Given the abundance of placental EVs in maternal circulation, these findings encourage immediate exploration of surface-enhanced Raman spectroscopy (SERS) of EVs as a promising method for preeclampsia liquid biopsies, while this novel fabrication process will provide a versatile and scalable substrate for many other SERS applications.

Detection of organic dyes by surface-enhanced Raman spectroscopy using plasmonic NiAg nanocavity films

This work presents the NiAg nanocavity film for the detection of organic dyes by surface-enhanced Raman spectroscopy (SERS). Nanocavity films were prepared by colloidal lithography using 518-nm polystyrene spheres combined with the electrochemical deposition of Ni supporting layer and Ag nanoparticles homogeneous SERS-active layer. The theoretical study was modelled by finite-difference time-domain (FDTD) simulation of electromagnetic field enhancement near the nanostructured surface and experimentally proven by SERS measurement of selected organic dyes (rhodamine 6G, crystal violet, methylene blue, and malachite green oxalate) in micromolar concentration. Furthermore, the concentration dependence was investigated to prove the suitability of NiAg nanocavity films to detect ultra-low concentrations of samples. The detection limit was 1.3 × 10^-12, 1.5 × 10^-10, 1.4 × 10^-10, 7.5 × 10^-11 mol·dm^-3, and the standard deviation was 20.1%, 13.8%, 16.7%, and 19.3% for R6G, CV, MB, and MGO, respectively. The analytical enhancement factor was 3.4 × 10⁵ using R6G as a probe molecule. The principal component analysis (PCA) was performed to extract the differences in complex spectra of the dyes where the first and second PCs carry 42.43% and 31.39% of the sample variation, respectively. The achieved results demonstrated the suitability of AgNi nanocavity films for the SERS-based detection of organic dyes, with a potential in other sensing applications.

Attomolar Sensing Based on Liquid Interface-Assisted Surface-Enhanced Raman Scattering in Microfluidic Chip by Femtosecond Laser Processing

Surface-enhanced Raman scattering (SERS) is a multidisciplinary trace analysis technique based on plasmonic effects. The development of SERS microfluidic chips has been exploited extensively in recent times impacting on applications in diverse fields. However, despite much progress, the excitation of label-free molecules is extremely challenging when analyte concentrations are lower than 1 nM because of the blinking SERS effect. In this paper, a novel analytical strategy which can achieve detection limits at an attomolar level is proposed. This performance improvement is due to the use of a glass microfluidic chip that features an analyte air-solution interface which forms on the SERS substrate in the microfluidic channel, whereby the analyte molecules aggregate locally at the interface during the measurement, hence the term liquid interface-assisted SERS (LI-SERS). The microfluidic chips are fabricated using hybrid femtosecond (fs) laser processing consisting of fs laser-assisted chemical etching, selective metallization, and metal surface nanostructuring. The novel LI-SERS technique can achieve an analytical enhancement factor of 1.5 × 10¹⁴, providing a detection limit below 10^-17 M (<10 aM). The mechanism for the extraordinary enhancement afforded by LI-SERS is attributed to Marangoni convection induced by the photothermal effect.