Surface-enhanced Raman Scattering Signal Amplification Research Articles

Gold-Trisoctahedra-Coated Capillary-Based SERS Platform for Microsampling and Sensitive Detection of Trace Fentanyl.

A cost-effective and highly reproducible capillary-based surface-enhanced Raman scattering (SERS) platform for sensitive, portable detection and identification of fentanyl is presented. Through encapsulating gold trisoctahedra (Au TOH) in the capillary tube for the first time, the SERS platform was constructed by combining the superior SERS properties of Au TOH and the advantages of capillaries in SERS signal amplification, facile sample extraction, and portable trace analysis. The effects of the size and density of Au TOH on the SERS performance were investigated by experiments and simulations, which showed that the maximum SERS enhancement was obtained for Au TOH with the size of 75 nm when particle density reached 74.54 counts/μm2. The proposed SERS platform possesses good reproducibility with a relative standard deviation (RSD) of less than 5%. As a demonstration, the platform was applied to detect fentanyl spiked in aqueous solution and serum samples with a limit of detection (LOD) as low as 1.86 and 40.63 ng/mL, respectively. We also validated the feasibility of the designed platform for accurate identification of trace fentanyl adulterated in heroin at mass concentration down to 0.1% (10 ng in 10 μg total). Overall, this work advances more explorations on capillary-based SERS platform to benefit portable trace analysis.

Gold-nanoparticle- and nanostar-loaded paper-based SERS substrates for sensing nanogram-level Picric acid with a portable Raman spectrometer

Nanoparticle (NP)-loaded filter paper (FP)-based surface-enhanced Raman scattering (SERS) substrates have been prepared using differently shaped gold (Au) NPs. The shape of Au NPs plays a significant role in the amplification of SERS signal. Here, two differently shaped Au NPs were synthesized using two different techniques: (a) femtosecond (fs) laser ablation in liquid and (b) chemical method. Spherical shaped Au NPs were obtained using fs ablation of a bulk Au target in distilled water and Au nanostars (NSs) were achieved through chemical process utilizing N-vinyl-2-pyrrolidone as a reducing/capping agent. The size and shapes of these synthesized NPs and NSs were investigated meticulously using different characterization techniques such as transmission electron microscopy, field emission scanning electron microscopy and X-ray diffraction. Both the NPs and NSs were subsequently loaded onto commercially available FP by simple drop casting method. To achieve higher number of hot spots, the aggregated spherical NPs were obtained by addition of NaCl. The non-aggregated spherical, aggregated spherical, and star Au NPs loaded on FP were used for the detection of a dye (Nile blue) and an explosive molecule (Picric acid).

Electrochromic semiconductors as colorimetric SERS substrates with high reproducibility and renewability

Electrochromic technology has been actively researched for displays, adjustable mirrors, smart windows, and other cutting-edge applications. However, it has never been proposed to overcome the critical problems in the field of surface-enhanced Raman scattering (SERS). Herein, we demonstrate a generic electrochromic strategy for ensuring the reproducibility and renewability of SERS substrates, which are both scientifically and technically important due to the great need for quantitative analysis, standardized production and low cost in SERS. This color-changing strategy is based on a unique quantitative relationship between the SERS signal amplification and the coloration degree within a certain range, in which the SERS activity of the substrate can be effectively inferred by judging the degree of color change. Our results may provide a first step toward the rational design of electrochromic SERS substrates with a high sensitivity, reproducibility, and renewability.

Ultrasensitive and Simultaneous Detection of Two Cytokines Secreted by Single Cell in Microfluidic Droplets via Magnetic-Field Amplified SERS.

A surface-enhanced Raman scattering (SERS)-microfluidic droplet platform for the rapid, ultrasensitive and simultaneous detection of vascular endothelial growth factor (VEGF) and interleukin-8 (IL-8) secreted by a single cell is presented. The high-throughput water-in-oil droplets containing an individual cell along with four kinds of immune-particles (antibody-conjugated silver nanoparticles or magnetic beads, AgNPs@Ab1 and MNs@Ab2) in each were achieved by a cross-typed microfluidic chip, and then they were captured by a collection channel array for SERS measurements. In the appearance of cytokines secreted by one cell, AgNPs@Ab1 can be linked onto the surface of MNs@Ab2 through the immune-recognition to form an immune-sandwich, which makes the "turn on" SERS signal of the Raman reporters previously laid on the surface of MNs due to the adjacent AgNPs. Furthermore, the second SERS signal amplification is from the magneticfield-induced spontaneous collection effect, which brings 75 times enhancement for SERS signal. Additionally, the encapsulation of cytokines in an isolated droplet permits an accumulation effect of targets with time. Owing to the dual signal enhancement and the accumulation effect, such few cytokines secreted by a single cell become detectable and a limit of detection is achieved as 1.0 fg/mL in one droplet. By using this ultrasensitive SERS-microdroplet method, the VEGF and IL-8 secretions from several cells in one droplet were explored and the data show that the cell-cell interaction may promote angiogenesis of cancer cells through the up-regulation of VEGF and IL-8.

Surface enhanced Raman scattering (SERS) based biomicrofluidics systems for trace protein analysis.

In recent years, Surface Enhanced Raman Scattering (SERS) has been widely applied to many different areas, including chemical analysis, biomolecule detection, bioagent diagnostics, DNA sequence, and environmental monitor, due to its capabilities of unlabeled fingerprint identification, high sensitivity, and rapid detection. In biomicrofluidic systems, it is also very powerful to integrate SERS based devices with specified micro-fluid flow fields to further focusing/enhancing/multiplexing SERS signals through molecule registration, concentration/accumulation, and allocation. In this review, after a brief introduction of the mechanism of SERS detection on proteins, we will first focus on the effectiveness of different nanostructures for SERS enhancement and light-to-heat conversion in trace protein analysis. Various protein molecule accumulation schemes by either (bio-)chemical or physical ways, such as immuno, electrochemical, Tip-enhanced Raman spectroscopy, and magnetic, will then be reviewed for further SERS signal amplification. The analytical and repeatability/stability issues of SERS detection on proteins will also be brought up for possible solutions. Then, the comparison about various ways employing microfluidic systems to register, concentrate, and enhance the signals of SERS and reduce the background noise by active or passive means to manipulate SERS nanostructures and protein molecules will be elaborated. Finally, we will carry on the discussion on the challenges and opportunities by introducing SERS into biomicrofluidic systems and their potential solutions.

Laser Processing of Low Optical Reflection Micro/Nano-patterned Si Substrates for SERS

ABSTRACTLight collection efficiency and specific molecular detection are crucial factors for the performance of bio-chemical molecule sensors. In this paper, the low optical-reflection silicon (Si) substrates which combine reduced optical reflectivity by light trapping effect and high Raman enhancement ability of gold nanoparticles (Au-NPs) coated textured Si substrates are investigated for surface-enhanced Raman scattering (SERS). A fast, single-step and highly controllable nanosecond (ns) laser processing technique is employed to fabricate textured Si substrates under ambient conditions. Parallel arrays of micro-pyramids are fabricated on Si surface by direct laser writing two-dimensional structures. SEM micrographs clearly show well-ordered surface features in the form of micro-pyramid shape with well-defined sharp tips on the laser processed Si substrates. The aggregation of Si micro/nanoparticles on Si surface forms nanocavities and nanogaps and further enhances the surface roughness in order to minimize the optical reflection. The low optical reflection Si substrates exhibit optical reflection below 15% over a broad wavelength range from 300 nm to 1200 nm. The textured Si substrates with high signal reproducibility are successfully applied as SERS substrates to detect a very small concentration of Rhodamine B molecules with an average enhancement factor of the order of ∼107. The low optical reflection and SERS signal amplification are also altered by the variation of laser pulse energy resulting into low optical reflection and high SERS signal intensity over the entire laser-patterned area. The effect of surface roughness on water contact angle was studied after the modification with Polydimethylsiloxane (PDMS), the surfaces show perfect superhydrophobicity with almost no water adhesion. This approach provides a novel high-speed and cost-effective method for fabricating SERS substrate with micro/nano-scale surfaces roughness and low optical reflection for high Raman signal enhancement.

A Hierarchical Nanostructure‐Based Surface‐Enhanced Raman Scattering Sensor for Preconcentration and Detection of Antibiotic Pollutants

A surface‐enhanced Raman scattering (SERS) sensor that integrates a plasmonic nanostructure with an electrostatic preconcentration unit has been developed. The silver nanoparticles (Ag‐NPs) grafted Ge nanowires (Ge‐NWs) are grown onto the surface of a commercial copper grid (Cu‐grid), which is usually used for transmission electron microscopy sample support. This leads to the formation of a hierarchical Ag‐NPs@Ge‐NWs@Cu‐grid substrate, which plays dual roles, i.e., preconcentration of analytes and amplification of SERS signal. This SERS detection platform based on the Ag‐NPs@Ge‐NWs@Cu‐grid substrate is able to selectively detect polar analytes such as antibiotic pollutants. This SERS sensor has achieved a limit of detection of 2.4 × 10−9m for 6‐aminopenicillanic acid and 0.9 × 10−9m for penicillin G, respectively. This work has demonstrated a promising SERS detection platform for monitoring polar SERS‐active pollutants in the aquatic environment.

‘Squeezed’ interparticle properties for plasmonic coupling and SERS characteristics of duplex DNA conjugated/linked gold nanoparticles of homo/hetero-sizes

The formation of interparticle duplex DNA conjugates with gold nanoparticles constitutes the basis for interparticle plasmonic coupling responsible for surface-enhanced Raman scattering signal amplification, but understanding of its correlation with interparticle spatial properties and particle sizes, especially in aqueous solutions, remains elusive. This report describes findings of an investigation of interparticle plasmonic coupling based on experimental measurements of localized surface plasmon resonance and surface enhanced Raman scattering characteristics for gold nanoparticles in aqueous solutions upon introduction of interparticle duplex DNA conjugates to define the interparticle spatial properties. Theoretical simulations of the interparticle optical properties and electric field enhancement based on a dimer model have also been performed to aid the understanding of the experimental results. The results have revealed a ‘squeezed’ interparticle spatial characteristic in which the duplex DNA-defined distance is close or shorter than A-form DNA conformation, which are discussed in terms of the interparticle interactions, providing fresh insight into the interparticle double-stranded DNA-defined interparticle spatial properties for the design of highly-sensitive nanoprobes in solutions for biomolecular detection.

Gold nanoparticle‐functionalized thread as a substrate for SERS study of analytes both bound and unbound to gold

The potential of thread for use as a substrate for inexpensive, disposable diagnostics for surface‐enhanced Raman scattering (SERS) spectroscopy has been showed in this study. Gold‐nanoparticle coated thread can be embedded into fabrics to detect chemical or biological analytes in military and medical applications through SERS. Using this inexpensive and widely available material enables reduction in the volumes of nanoparticle solution required compared to alternatives. By testing multiple analytes, it was observed that molecular structure played a significant role in SERS signal amplification, and hence, the technique is limited to the detection of a small number of analytes possessing highly polarizable structures. Although direct chemical bonding between analyte molecules and nanoparticles gives the strongest signal enhancement, it remains possible to easily discern signals generated by analytes not directly bound, provided they possess suitable structure. Amplification of SERS signal by controlling the aggregation state of the gold nanoparticles to increase the number of SERS hotspots was observed. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1598–1605, 2014