Highly sensitive and simple SERS substrate based on photochemically generated carbon nanotubes–gold nanorods hybrids

Oriented gold nanorods on polyacrylic acid grafted carbon nanotube for efficient SERS detection of cationic dyes

In this work, we successfully achieved controllable assembly of gold nanorods (AuNRs) into end-to-end orientation using polyacrylic acid (PAA) grafted carbon nanotube (CNT) (i.e., CNT-g-PAA) as a template. The resulting CNT-g-PAA/AuNRs hybrid assemblies were prepared via electrostatic interactions between negnative PAA-grafted CNT and positive hexadecyltrimethylammonium bromide (CTAB)-capped AuNRs. Due to the anisotropic nature of AuNRs and one-dimensional structure of CNT, the end-to-end arrangement of AuNRs immobilized on the CNT-g-PAA results in enhanced uniaxial plasmon coupling and thus stronger SERS response, as compared to the 3D SiO2-g-PAA/AuNRs assemblies. In addition, PAA-functionalized CNTs ensure the predominantly end-to-end assembly of AuNRs on the CNT surface, which greatly enhances the structural stability of the assemblies. Moreover, the PAA brushes on the CNT surface contribute to the selective and efficient adsorption of cationic dyes on the SERS substrate surface. This enhancement has improved the sensitivity of the substrate, achieving detection limits for malachite green (MG) and crystal violet (CV) as low as 10-10 M. Our present strategy opens up an avenue for the fabrication of novel optically enhanced nanodevices.

Gold nanoparticles for surface-enhanced Raman scattering detection of benzyldimethyldodecylammonium chloride at low concentration

The oil and gas industry plays a vital role in the global economy. The production process has several critical conditions and can expose metals to corrosion. Surfactants like the quaternary ammonium salt Benzyldimethyldodecylammonium Bromide (BDAC) are currently used to prevent corrosions; classical methods for determining these surfactants have problems in saline samples and usually present high costs. In this context, spectroscopic techniques become an excellent alternative for quaternary ammonium salts detection. Here, a SERS (surface-enhanced Raman scattering) sensor based on gold nanoparticles (AuNPs) synthesized through chemical reduction was used as an alternative method for BDAC detection. We detected BDAC at low concentrations in water solutions: at 5 to 30 ppm (1.47 × 10-5 mol L-1 to 8.82 × 10-5 mol L-1); and had the vibration attempt attribute analyzed. A new study of quaternary ammonium compounds using AuNPs and SERS with a different, easy, and repeatable approach to spectra acquisition is presented and shows to be a promising method applied in quaternary ammonium salt compounds detection for the oil and gas industry.

Enhancing in vitro photothermal therapy using plasmonic gold nanorod decorated multiwalled carbon nanotubes

Photothermal therapy (PTT) is a promising approach for cancer treatment that selectively heats malignant cells while sparing healthy cells. Here, the light-to-heat conversion efficiency of multiwalled carbon nanotubes (MWCNTs) within the near-infrared biological transmission window is enhanced by decorating them with plasmonic gold nanorods (GNRs). The results reveal a significant photothermal enhancement of hybrid MWCNTs-GNRs compared to bare MWCNTs, displaying a 4.9 enhancement factor per unit mass. The enhanced plasmonic PTT properties of MWCNTs-GNRs are also investigated in vitro using PC3 prostate cancer cell lines, demonstrating a potent ablation efficiency. These findings advance innovative hybrid plasmonic nanostructures for clinical applications.

Dimensional Design for Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopy technique that enables specific identification of target analytes with sensitivity down to the single-molecule level by harnessing metal nanoparticles and nanostructures. Excitation of localized surface plasmon resonance of a nanostructured surface and the associated huge local electric field enhancement lie at the heart of SERS, and things will become better if strong chemical enhancement is also available simultaneously. Thus, the precise control of surface characteristics of enhancing substrates plays a key role in broadening the scope of SERS for scientific purposes and developing SERS into a routine analytical tool. In this review, the development of SERS substrates is outlined with some milestones in the nearly half-century history of SERS. In particular, these substrates are classified into zero-dimensional, one-dimensional, two-dimensional, and three-dimensional substrates according to their geometric dimension. We show that, in each category of SERS substrates, design upon the geometric and composite configuration can be made to achieve an optimized enhancement factor for the Raman signal. We also show that the temporal dimension can be incorporated into SERS by applying femtosecond pulse laser technology, so that the SERS technique can be used not only to identify the chemical structure of molecules but also to uncover the ultrafast dynamics of molecular structural changes. By adopting SERS substrates with the power of four-dimensional spatiotemporal control and design, the ultimate goal of probing the single-molecule chemical structural changes in the femtosecond time scale, watching the chemical reactions in four dimensions, and visualizing the elementary reaction steps in chemistry might be realized in the near future.

SERS-Based Methodology for the Quantification of Ultratrace Graphene Oxide in Water Samples

The extensive use of graphene materials in real-world applications has increased their potential release into the environment. To evaluate their possible health and ecological risks, there is a need for analytical methods that can quantify these materials at very low concentrations in environmental media such as water. In this work, a simple, reproducible, and sensitive method to detect graphene oxide (GO) in water samples using the surface-enhanced Raman spectroscopy (SERS) technique is presented. The Raman signal of graphene is enhanced when deposited on a substrate of gold nanoparticles (AuNPs), thus enabling its determination at low concentrations with no need for any preconcentration step. The practical limit of quantification achieved with the proposed method was 0.1 ng mL^-1, which is lower than the predicted concentrations for graphene in effluent water reported to date. The optimized procedure has been successively applied to the determination of ultratraces of GO in water samples.

Differential surface partitioning for an ultrasensitive solid-state SERS sensor and its application to food colorant analysis

Solid-state SERS sensors are desirable point-of-care tools due to their portability. However, the level of SERS sensitivity achieved in liquid phase is rarely duplicated in the solid phase. We report herein the fabrication of a SERS sensor using alumina beads as the solid support and demonstrate its high SERS sensitivity with the model analyte 4-aminophenyl disulfide (4-APDS). The key to sensitivity is a hydrophilic-hydrophobic surface gradient constructed by sequentially coating with the surfactant cetyltrimethylammonium bromide and fluorous 1H,1H,2H,2H-perfluorooctyltriethoxysilane. The surface gradient, together with chloride etching, allows the detection of 4-APDS at the low concentration of 10^-15 M. The practicality of the sensor beads is evidenced by successfully tracking the SERS fingerprints of five food colorant standards in the SERS spectra of a popular candy product. These SERS sensor beads are easy to prepare, convenient to use, and highly responsive as a SERS platform for the analysis of colorants.

Electrospun polymeric nanofiber decorated with sea urchin-like gold nanoparticles as an efficient and stable SERS platform

Surface enhanced Raman scattering (SERS)-based nanoprobes have been used as well-established analytical tools enabling single-molecule detection. In this work, we report a facile method to decorate sea urchin-like gold nanoparticles (SUGNPs) on the surface of PMMA/P4VP nanofibers. Firstly, PMMA/P4VP nanofibers within the submicrometer size range were prepared by applying the electrospinning technique. Then, the incorporation of SUGNPs on the surface of PMMA/P4VP nanofiber was achieved by immersing PMMA/P4VP nanofiber into freshly prepared SUGNP aqueous solution through the specific Au-N interactions. The as-fabricated SUGNP-coated PMMA/P4VP nanofibers exhibited good sensitivity and reproducibility in SERS measurements with the relative standard deviation down to 6.6%, by employing 4-mercaptobenzoic acid as a probe molecule with 30 min of soaking time. Hence, we envisage that the SUGNP-coated PMMA/P4VP nanofibers can act as efficient and stable SERS substrates for potential applications in molecular detection as well as chemical and biological analysis.

Toward the highly sensitive SERS detection of bio-molecules: the formation of a 3D self-assembled structure with a uniform GO mesh between Ag nanoparticles and Au nanoparticles

We report a structure to form a hybrid system in which a mesh is sandwiched between Au nanoparticles (AuNPs) and Ag nanoparticles (AgNPs). This self-assembly method uses smaller and denser AgNPs "hot spots" that are spin-coated on a AuNPs@GO mesh nanostructure formed by the reaction of GO@MoS₂ and HAuCl₄ to form AuNPs@GO mesh@AgNPs SERS substrates. Sub-40-nm mesh and 10-nm gaps ensure the landing sites and spacing of the AgNPs. Consequently, the design integrates the strong plasmonic effects of AgNPs and AuNPs with the biological compatibility of the GO mesh. Crystal violet (CV) as low as 10^-15 M can be detected, which confirms the ultrahigh sensitivity of AuNPs@GO mesh@AgNPs. Furthermore, the reproducibility, stability, and finite-difference time-domain (FDTD) simulations confirm the value of this SERS substrate. This material can be used for label-free DNA detection, and the AuNPs@GO mesh@AgNPs substrate facilitated single-molecule DNA detection limits.

In situ detection of trace pollutants: a cost-effective SERS substrate of blackberry-like silver/graphene oxide nanoparticle cluster based on quick self-assembly technology

To realize fast detection of trace hazardous chemicals, a SERS substrate with the structure of a blackberry-like silver/graphene oxide nanoparticle cluster (Ag/GO NPC) has been designed and prepared through a quick capillarity-assistant self-assembly technology in this paper. Benefitting from the abundant "hot spots" and active oxygen sites brought by this Ag/GO NPC, the substrate shows good Raman performance for malachite green (MG), a common abusive germicide in aquaculture, with lowest limit of detection below 0.1 µg/L (3.48 × 10^-10 mol/L). Detailed analyses are taken on both the formation process and enhancement mechanism of this SERS substrate, and the finite-difference time-domain simulations are utilized as well to prove our hypotheses. Further constructing this structure on polyethylene terephthalate (PET) film, a translucent flexible SERS substrate can be obtained, realizing a fast in situ detection of trace MG in the fishpond subsequently. In consideration of the facile preparation process, good SERS enhancement and affordable materials (PET, Cu, Ag and GO, etc.), this substrate presents high cost performance and a promising application prospect.

Nanostructured hybrid surface enhancement Raman scattering substrate for the rapid determination of sulfapyridine in milk samples

The fabrication of surface-enhanced Raman spectroscopy (SERS) substrates, which can offer the advantages of strong Raman signal enhancement with good reproducibility, is still a challenge for practical applications. In this work, a simple and reproducible SERS substrate combining the properties of multi-walled carbon nanotubes (MWCNTs) and gold nanoparticles (AuNPs), is proposed for the determination and quantification of sulfapyridine in milk samples with a concentration range of 10-100 ng mL^-1. The Raman signals of sulfapyridine is enhanced at factor of 4394. The procedure presented is capable of detecting and quantifying small quantities of sulfapyridine without implying any preconcentration step, just using an affordable and portable Raman spectrometer. The precision, in terms of repeatability and inter and intermediate precision, was lower than 8% in all cases.

Novel Strategy for One-Pot Synthesis of Gold Nanoplates on Carbon Nanotube Sheet As an Effective Flexible SERS Substrate

In this work, we demonstrate a novel route for one-pot synthesis of two-dimensional gold nanoplates (2-D AuNPLs) on carbon nanotube (CNT) sheet. Well-defined AuNPLs are grafted onto CNT sheet via a facile hydrothermal reduction process, during which bromine ions are employed as the surfactant for gold anisotropic growth. Scanning electron microscopy (SEM) shows large-scale AuNPLs with micrometer-scaled length and sub-100 nm thickness are deposited uniformly on the CNT sheet. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) results confirm the synthesized AuNPLs are single-crystalline with preferential {111} orientation. Based on the CNT sheet/AuNPLs hybrid, we have fabricated a flexible surface-enhanced Raman scattering (SERS) substrate, which can effectively detect the analyte Rhodamine 6G (Rh6G) at the concentration as low as 1 × 10^-7 M. The excellent SERS performance of this novel flexible substrate is mainly attributed to nanoscaled gaps between the neighbors, large surface area with roughness, and their sharp edges and corners.

Single wall carbon nanotube (SWCNT)–gold nanorod (AuNR) conjugates via thermally-mild reaction conditions

A thermally-mild method for covalent binding of SWCNTs to AuNRs, based on an inverse-electron-demand Diels–Alder reaction, is established and discussed.