Xiaokun Wang

Highly sensitive surface-enhanced Raman scattering sensing of heparin based on antiaggregation of functionalized silver nanoparticles.

We report a simple and sensitive surface-enhanced Raman scattering (SERS) platform for the detection of heparin, based on antiaggregation of 4-mercaptopyridine (4-MPY) functionalized silver nanoparticles (Ag NPs). Here, protamine was employed as a medium for inducing the aggregation of negatively charged 4-MPY functionalized Ag NPs through surface electrostatic interaction, which resulted in significantly enhanced Raman signal of the Raman reporter. However, in the presence of heparin, the interaction between heparin and protamine decreased the concentration of free protamine, which dissipated the aggregated 4-MPY functionalized Ag NPs and thus decreased Raman enhancement effect. The degree of aggregation and Raman enhancement effect was proportional to the concentration of added heparin. Under optimized assay conditions, good linear relationship was obtained over the range of 0.5-150 ng/mL (R(2) = 0.998) with a minimum detectable concentration of 0.5 ng/mL in standard aqueous solution. Furthermore, the developed method was also successfully applied for detecting heparin in fetal bovine serum samples with a linear range of 1-400 ng/mL.

Ultrasensitive surface-enhanced Raman scattering nanosensor for mercury ion detection based on functionalized silver nanoparticles

In this work, a simple, rapid and ultrasensitive surface-enhanced Raman scattering (SERS) nanosensor was developed for mercury ion (Hg2+) detection based on 4-mercaptopyridine (4-MPY) functionalized silver nanoparticles (AgNPs) (4-MPY-AgNPs) in the presence of spermine. Here, the spermine would bind AgNPs through Ag–N bonds and induce remarkable aggregation of AgNPs, and thereby would generate significantly enhanced Raman intensity of the reporter molecule 4-MPY. Followed by the addition of Hg2+, the formation of the Hg–Ag alloy blocked the adsorption of 4-MPY and spermine, resulting in the dispersion of 4-MPY-AgNPs and thus decreasing the Raman intensity, by which the Hg2+ could be sensed by SERS. A good linearity was obtained in the range of 1–100 nM (R2 = 0.987), and the relative standard deviation was between 0.85 and 5.50%. The spermine-induced accumulation of 4-MPY-AgNPs largely enhanced the SERS responses, leading to a high detectability up to 0.34 nM. A real water sample with spiked Hg2+ was also analyzed, presenting satisfactory recoveries ranging from 94.5 to 108.5%, confirming the practicability of the SERS nanosensor based method.

Simultaneous Detection of Dual Nucleic Acids Using a SERS-Based Lateral Flow Assay Biosensor

A new class of surface-enhanced Raman scattering (SERS)-based lateral flow assay (LFA) biosensor has been developed for the simultaneous detection of dual DNA markers. The LFA strip in this sensor was composed of two test lines and one control line. SERS nano tags labeled with detection DNA probes were used for quantitative evaluation of dual DNA markers with high sensitivity. Target DNA, associated with Kaposis sarcoma-associated herpesvirus (KSHV) and bacillary angiomatosis (BA), were tested to validate the detection capability of this SERS-based LFA strip. Characteristic peak intensities of SERS nano tags on two test lines were used for quantitative evaluations of KSHV and BA. The limits of detection for KSHV and BA, determined from our SERS-based LFA sensing platform, were estimated to be 0.043 and 0.074 pM, respectively. These values indicate approximately 10 000 times higher sensitivity than previously reported values using the aggregation-based colorimetric method. We believe that this is the first report of simultaneous detection of two different DNA mixtures using a SERS-based LFA platform. This novel detection technique is also a promising multiplex DNA sensing platform for early disease diagnosis.

Sensitive and Reproducible Immunoassay of Multiple Mycotoxins Using Surface-Enhanced Raman Scattering Mapping on 3D Plasmonic Nanopillar Arrays

A surface-enhanced Raman scattering-based mapping technique is reported for the highly sensitive and reproducible analysis of multiple mycotoxins. Raman images of three mycotoxins, ochratoxin A (OTA), fumonisin B (FUMB), and aflatoxin B1 (AFB1) are obtained by rapidly scanning the surface-enhanced Raman scattering (SERS) nanotags-anchoring mycotoxins captured on a nanopillar plasmonic substrate. In this system, the decreased gap distance between nanopillars by their leaning effects as well as the multiple hot spots between SERS nanotags and nanopillars greatly enhances the coupling of local plasmonic fields. This strong enhancement effect makes it possible to perform a highly sensitive detection of multiple mycotoxins. In addition, the high uniformity of the densely packed nanopillar substrate minimizes the spot-to-spot fluctuations of the Raman peak intensity in the scanned area when Raman mapping is performed. Consequently, this makes it possible to gain a highly reproducible quantitative analysis of mycotoxins. The limit of detections (LODs) are determined to be 5.09, 5.11, and 6.07 pg mL-1 for OTA, FUMB, and AFB1, and these values are approximately two orders of magnitude more sensitive than those determined by the enzyme-linked immunosorbent assays. It is believed that this SERS-based mapping technique provides a facile tool for the sensitive and reproducible quantification of various biotarget molecules.

Biomedical Applications of Surface-Enhanced Raman Scattering Spectroscopy

Abstract Many efforts have been invested in the development of rapid and sensitive detection methods for the quantification of disease biomarkers in human blood. Although luminescence- and fluorescence-based detection methods, combined with an automatic sampling system, are routinely used for immunoassays of specific biomarkers, a more sensitive detection technique is needed to track disease progression in its early stage. Recently, a surface-enhanced Raman scattering (SERS)-based immunoassay technique has been considered as a strong candidate to resolve the problem of low sensitivity in conventional luminescence- or fluorescence-based clinical immunoassays. Due to its highly sensitive detection capability and the feasibility of automatic assay, the SERS-based assay technique has strong potential to substitute for currently available fluorescence or luminescence assay systems in clinical laboratory settings. To date, a variety of clinical biomarkers such as proteins, DNAs, hormones, viruses, bacteria, and toxins have been measured using the SERS-based assay technique. Several different types of assay platform have been developed for rapid and reproducible SERS-based assays. In this chapter, four different SERS-based assay platforms—gold-patterned microarray-type substrates, magnetic bead-based assay platforms, microfluidic devices, and lateral flow assay platforms—will be introduced. In addition, the diagnostic feasibility of these assay platforms for various biomarkers will be described.

Simultaneous immunoassays of dual prostate cancer markers using a SERS-based microdroplet channel.

The simultaneous detection of multiple biomarkers plays an important role in the accurate diagnosis of cancer. In this study, we developed a novel, surface-enhanced Raman scattering (SERS)-based microfluidic device for the simultaneous detection of free prostate-specific antigen (f-PSA) and total PSA (t-PSA) markers. A fully automatic droplet-based microfluidic platform for the rapid and sensitive detection of f-PSA and t-PSA was designed. Magnetic immunocomplexes were aligned on one side of the channel using a permanent magnet embedded in the microfluidic device, and parent microdroplets containing magnetic immunocomplexes and supernatant solutions were split into two smaller daughter droplets at the Y-shaped junction of the channel. Then, Raman signals of sequential droplets including supernatant solutions were measured for the quantitative analysis of the PSA markers. Two parallel microfluidic channels were designed and fabricated for the simultaneous detection of f-PSA and t-PSA. Our results showed a good linear response for both PSA markers in the range from 0.05 to 100 ng mL-1. The limits of detection were estimated to be below 0.1 ng mL-1 for both the f-PSA and t-PSA. This SERS-based assay in a microfluidic channel was completed in 10 min without any manual incubation and washing steps. Our method is a very promising clinical tool for PSA-based screening test of prostate cancer.

Culture-Free Detection of Bacterial Pathogens on Plasmonic Nanopillar Arrays Using Rapid Raman Mapping

We utilized a fast Raman spectral mapping technique for fast detection of bacterial pathogens. Three-dimensional (3D) plasmonic nanopillar arrays were fabricated using the nanolithography-free process consisting of maskless Ar plasma treatment of a polyethylene terephthalate substrate and subsequent metal deposition. Bacterial pathogens were immobilized on the positively charged poly(l-lysine)-coated 3D plasmonic substrate through electrostatic interactions. Then, the bacterial surfaces were selectively labeled with antibody-conjugated surface-enhanced Raman scattering (SERS) nanotags, and Raman mapping images were collected and statistically analyzed for quantitative analysis of bacteria. Salmonella typhimurium was selected as a model pathogen bacterium to confirm the efficacy of our SERS imaging technique. Minimum number of Raman mapping points with statistical reliability was determined to reduce assay time. It was possible to get a statistically reliable standard calibration curve for 529 pixels (laser spot with 60 μm interval), which required a total mapping time of 45 min to get a standard calibration curve for five different concentrations of bacteria in the 0 to 106 CFU/mL range. No amplification step was necessary for quantification because low-abundance target bacteria could be measured using the Raman spectral mapping technique. Therefore, this approach allows accurate quantification of bacterial pathogens without any culturing or enrichment process.