Detection of proteins on silica-silver core-shell substrates by surface-enhanced Raman spectroscopy

A spectroscopic sensing platform for MARCKS protein monolayers

We developed a highly sensitive silicon platform, suitable to assess the molecular organization of protein samples. Prototype platforms were obtained using different electrochemical protocols for the electrodeposition of Ag-nanoparticles onto the hydrogenated silicon surface. A platform with high Surface Enhanced Raman Scattering efficiency was selected based on the surface coverage and the number density of particles size distribution. The performance of the platform was determined by studying the interaction of Myristoylated Alanine-Rich C Kinase Substrate (MARCKS) protein with the substrate according to its molecular organization. The chemical and structural characteristics of MARCKS molecules were examined under two configurations: i) a disordered distribution given by a MARCKS solution drop deposited onto the platform and, ii) a compact monolayer transferred to the platform by the Langmuir-Blodgett method. Raman spectra show vibrational bands of Phenylalanine and Lysine residues specific for the protein effector domain, and evidence the presence of alpha helix structure in both configurations. Moreover, we distinguished the supramolecular order between the compact monolayer and random molecular distribution. The platforms containing Ag-nanoparticles are suitable for studies of protein structure and interactions, advancing a methodological strategy for our long term goal, which is to explore the interaction of proteins with model membranes.

Surface-Enhanced Raman Scattering (SERS) Active Gold Nanoparticles Decorated on a Porous Polymer Filter

In this work, we designed a process to assemble gold nanoparticles onto a three-dimensional (3D) polymer surface, which can then be monitored using surface-enhanced Raman scattering (SERS). This work is the first demonstration of the assembly of gold nanoparticles on a filter film and in situ measurement with Raman spectroscopy. Herein, a polyhexamethylene adipamide (Nylon66) film embedded in the organic filter film was used as a template to fabricate a tunable SERS-active substrate. A "hotspot"-rich gold-nanoparticle-decorated polymer substrate for SERS was prepared; this substrate exhibited high sensitivity in trace detection of targets. The study was conducted using 4-mercaptobenzoic acid as a probe molecule with the aim of comparing the scattering efficiency and the homogeneity of the Raman signal on selected substrates. In addition, we used the gold-decorated polymer film to detect a biotin-avidin complex. The most powerful advantage of the proposed microanalytical device is the in situ SERS application. The 3D nanoporous structures described in this work hold strong potential for use in various applications such as environmental monitoring and biomolecule detection.

Highly uniform SERS-active microchannel on hydrophobic PDMS: a balance of high reproducibility and sensitivity for detection of proteins

SERS-active microchannels on hydrophobic polydimethylsiloxane (PDMS) with both high reproducibility and sensitivity are fabricated. Proteins might denature while drying on the SERS-active substrate, but keep native structures in the microchannels.

Nanostructure-based surface-enhanced Raman scattering biosensors for nucleic acids and proteins

Detection of nucleic acid and protein targets related to human health and safety has attracted widespread attention. Surface-enhanced Raman scattering (SERS) is a powerful tool for biomarker detection because of its ultrahigh detection sensitivity and unique fingerprinting spectra. In this review, we first introduce the development of nanostructure-based SERS-active substrates and SERS nanotags, which greatly influence the performance of SERS biosensors. We then focus on recent advances in SERS biosensors for DNA, microRNA and protein determination, including label-free, labeled and multiplex analyses as well as in vivo imaging. Finally, the prospects and challenges of such nanostructure-based SERS biosensors are discussed.

SERS-based immunoassay using a core–shell SiO2@Ag immune probe and Ag-decorated NiCo2O4 nanorods immune substrate

A novel sandwich structure consisting of the SiO₂@Ag immune probe and the Ag-decorated NNRs substrate was used to detect AFP and a detection limit is as low as 2.1 fg mL⁻¹.

Mercury species induced frequency-shift of molecular orientational transformation based on SERS

We proposed a novel readout method based on a peculiar phenomenon in which the vibrational frequencies of a SERS-active probe (dimethyldithiocarbamic acid sodium salt, DASS) can be affected when there is mercury species.

Probing cytochrome c in living mitochondria with surface-enhanced Raman spectroscopy

Selective study of the electron transport chain components in living mitochondria is essential for fundamental biophysical research and for the development of new medical diagnostic methods. However, many important details of inter- and intramembrane mitochondrial processes have remained in shadow due to the lack of non-invasive techniques. Here we suggest a novel label-free approach based on the surface-enhanced Raman spectroscopy (SERS) to monitor the redox state and conformation of cytochrome c in the electron transport chain in living mitochondria. We demonstrate that SERS spectra of living mitochondria placed on hierarchically structured silver-ring substrates provide exclusive information about cytochrome c behavior under modulation of inner mitochondrial membrane potential, proton gradient and the activity of ATP-synthetase. Mathematical simulation explains the observed enhancement of Raman scattering due to high concentration of electric near-field and large contact area between mitochondria and nanostructured surfaces.

One-step synthesis of silver nanoshells with bumps for highly sensitive near-IR SERS nanoprobes

A seedless, one-step synthetic route to uniform bumpy silver nanoshells (AgNSs) as highly NIR sensitive SERS substrates is reported. These substrates can incorporate Raman label compounds and biocompatible polymers on their surface. AgNS based NIR-SERS probes are successfully applied to cell tracking in a live animal using a portable Raman system.

Using a silver-enhanced microarray sandwich structure to improve SERS sensitivity for protein detection

A simple and sensitive method, based on surface-enhanced Raman scattering (SERS), for immunoassay and label-free protein detection is reported. A series of bowl-shaped silver cavity arrays were fabricated by electrodeposition using a self-assembled polystyrene spheres template. The reflection spectra of these cavity arrays were recorded as a function of film thickness, and then correlated with SERS enhancement using sodium thiophenolate as the probe molecule. The results reveal that SERS enhancement can be maximized when the frequency of both the incident laser and the Raman scattering approach the frequency of the localized surface plasmon resonance. The optimized array was then used as the bottom layer of a silver nanoparticle-protein-bowl-shaped silver cavity array sandwich. The second layer of silver was introduced by the interactions between the proteins in the middle layer of the sandwich architecture and silver nanoparticles. Human IgG bound to the surface of this microcavity array can retain its recognition function. With the Raman reporter molecules labeled on the antibody, a detection limit down to 0.1 ng mL(-1) for human IgG is easily achieved. Furthermore, the SERS spectra of label-free proteins (catalase, cytochrome C, avidin and lysozyme) from the assembled sandwich have excellent reproducibility and high quality. The results reveal that the proposed approach has potential for use in qualitative and quantitative detection of biomolecules.

Surface-enhanced Raman scattering: an emerging label-free detection and identification technique for proteins

The detection and identification of biologically important molecules has critical importance in several fields such as medicine, biotechnology, and pharmacology. Surface-enhanced Raman scattering (SERS) is a powerful emerging vibrational spectroscopic technique that allows not only for the characterization, but also for the identification and detection of biomacromolecules in a very short time. In this review, efforts to utilize SERS for label-free protein detection and identification is summarized after a short introduction of proteins and the technique.

Functional hybrid nickel nanostructures as recyclable SERS substrates: detection of explosives and biowarfare agents

We present the synthesis of highly anisotropic nickel nanowires (NWs) and large area, free-standing carpets extending over cm(2) area by simple solution phase chemistry. The materials can be post-synthetically manipulated to produce hybrid tubes, wires, and carpets by galvanic exchange reactions with Au(3+), Ag(+), Pt(2+), and Pd(2+). All of these structures, especially the hybrid carpets and tubes, have been prepared in bulk and are surface enhanced Raman scattering (SERS) active substrates. Molecules of relevance such as dipicolinic acid (constituting 5-15% of the dry weight of bacterial spores of Bacillus anthracis), dinitrotoluene, hexahydro-1,3,5-triazine (RDX), and trinitrotoluene at nanomolar concentrations have been detected. An enhancement factor of ∼10(10) was observed for the Ni-Au nanocarpet. The reusability of the Ni-Au nanocarpet for SERS applications was tested 5 times without affecting the sensitivity. The reusability and sensitivity over large area have been demonstrated by Raman microscopy. Our method provides an easy and cost effective way to produce recyclable, large area, SERS active substrates with high sensitivity and reproducibility which can overcome the limitation of one-time use of traditional SERS substrates.

Ag-SiO2 core-shell nanorod arrays: morphological, optical, SERS, and wetting properties

Using the hydrolysis of tetraethylorthosilicate, a uniform and conformal layer of porous SiO(2) with controlled thickness has been coated onto the oblique angle deposited Ag nanorod (AgNR) array to form an aligned AgNR-SiO(2) core-shell array nanostructure. The morphology, optical property, SERS response, and surface wettability of the AgNRs with different SiO(2) shell thicknesses have been obtained by multiple characterization techniques. The morphological characterization shows that each AgNR on the array is coated with a uniform and porous silica shell independently and the growth of shell thickness follows a linear function versus the coating time. Thickening of the shell induces a monotonic decrease of the apparent contact angle, red-shift of the transverse mode of the localized surface plasmon resonance peak, and makes the SiO(2) shell more compact. The SERS response of 4-Mercaptophenol on these substrates exhibits an exponential decay behavior with the increasing coating time, which is ascribed to the decreasing Ag surface coverage of core-shell nanorods. Under the assumption that the Ag surface coverage is proportional to the SERS intensity, one can estimate the evolution of SiO(2) coverage on AgNRs. Such coverage evolution can be used to qualitatively explain the LSPR wavelength change and quantitatively interpret the contact angle change based on a double Cassie's law.