Deep Eutectic Solvent-Assisted Synthesis of Au Nanostars Supported on Graphene Oxide as an Efficient Substrate for SERS-Based Molecular Sensing

A novel strategy for specific sensing and inactivation of Escherichia coli: Constructing a targeted sandwich-type biosensor with multiple SERS hotspots to enhance SERS detection sensitivity and near-infrared light-triggered photothermal sterilization performance

Human health is greatly threatened by bacterial infection, which raises the risk of serious illness and death in humans. For early screening and accurate treatment of bacterial infection, there is a strong desire to undertake ultrasensitive detection and effective killing of pathogenic bacteria. Herein, a novel surface-enhanced Raman scattering (SERS) biosensor based on sandwich structure consisting of capture probes/bacteria/SERS tags was established for specific identification, capture and photothermal killing of Escherichia coli (E. coli). Finite-difference time-domain (FDTD) technique was used to simulate the electromagnetic field distribution of capture probes, SERS tags and sandwich-type SERS substrate, and a possible SERS enhancement mechanism based on sandwich structure was presented and discussed. Sandwich-type SERS biosensor successfully achieved distinctive identification and magnetic beneficiation of E. coli. In addition, a single SERS substrate, including capture probes and SERS tags, could also achieve outstanding photothermal effects as a consequence of localized surface plasmon resonance (LSPR) effect. Intriguingly, sandwich-type SERS biosensor demonstrated a higher photothermal conversion efficiency (50.03 %) than the single substrate, which might be attributed to the formation of target bacterial clusters. The superior biocompatibility and the low toxicity of the sandwich-type biosensor were confirmed. Our approach offers a fresh method for constructing sandwich-type biosensor with multiple SERS hotspots based on extremely effective hybrid plasmonic nanoparticles, and has a wide range of potential applications in the recognition and treatment of bacteria.

Biomass-assisted fabrication of rGO-AuNPs as surface-enhanced Raman scattering substrates for in-situ monitoring methylene blue degradation

Reduced graphene oxide-gold nanoparticles nanocomposites (rGO-AuNPs) with high surface-enhanced Raman scattering (SERS) activity was created by biomass-assisted green synthesis with Lilium casa blanca petals biomass for the first time, and its application for methylene blue (MB) degradation was explored through in-situ monitoring. Lilium casa blanca petals biomass was used as a reducing agent to reduce GO and chloroauric acid successively when carrying out rGO-AuNPs in-situ synthesis while it also acted as a capping agent. The produced rGO had oxygen-containing functional groups which had an outstanding performance in enhancing the SERS effect. Characterization results confirmed that the AuNPs were grafted onto the rGO sheet, and the mechanism study showed that total flavonoids in Lilium casa blanca petals biomass were the main biological compounds involved in the reduction. rGO-AuNPs had a high Raman enhancement factor (EF) which could reach 3.88 × 10⁷. The synthesized nanocomposite also had a good catalytic activity that could be employed as catalyst in MB degradation, and it could complete degradation within 15min. The reaction rate increased linearly with the amount of rGO-AuNPs, and the degradation could be in-situ monitored both by UV and SERS.

Ultrasensitive SERS substrate for label-free therapeutic drug monitoring of chlorpromazine hydrochloride and aminophylline in human serum

Surface-enhanced Raman spectroscopy (SERS) has been widely used in the field of therapeutic drug monitoring (TDM) because of its powerful fingerprinting capability. In this paper, we used an in situ synthesis method to anchor Ag nanoparticles (AgNPs) on the surface of MIL-101(Cr) to obtain MIL-101(Cr)@Ag. Owing to the large specific surface area and ultra-high porosity of MIL-101(Cr)@Ag, we developed a method for the determination of chlorpromazine hydrochloride (CPZ) and aminophylline (AMP) in human serum by using it as a solid-phase extraction sorbent and SERS substrate. The label-free TDM-SERS method was able to evaluate the levels of CPZ and AMP in serum samples with detection limits as low as 8.91 × 10^-2 µg/mL and 3.4 × 10^-2 µg/mL, respectively. In addition, influencing factors including sample solution pH, AgNO₃ concentration, drug adsorption time, and the amount of sample solution were optimized. This protocol provides a new method with good selectivity, stability, reproducibility, homogeneity, and sensitivity for the determination of small-molecule drug content in serum samples. This label-free TDM-SERS method will help to achieve rapid individualized dosing regimens in clinical practice and has potential applications in the field of TDM.

Real-time SERS monitoring anticancer drug release along with SERS/MR imaging for pH-sensitive chemo-phototherapy

In situ and real-time monitoring of responsive drug release is critical for the assessment of pharmacodynamics in chemotherapy. In this study, a novel pH-responsive nanosystem is proposed for real-time monitoring of drug release and chemo-phototherapy by surface-enhanced Raman spectroscopy (SERS). The Fe3O4@Au@Ag nanoparticles (NPs) deposited graphene oxide (GO) nanocomposites with a high SERS activity and stability are synthesized and labeled with a Raman reporter 4-mercaptophenylboronic acid (4-MPBA) to form SERS probes (GO-Fe3O4@Au@Ag-MPBA). Furthermore, doxorubicin (DOX) is attached to SERS probes through a pH-responsive linker boronic ester (GO-Fe3O4@Au@Ag-MPBA-DOX), accompanying the 4-MPBA signal change in SERS. After the entry into tumor, the breakage of boronic ester in the acidic environment gives rise to the release of DOX and the recovery of 4-MPBA SERS signal. Thus, the DOX dynamic release can be monitored by the real-time changes of 4-MPBA SERS spectra. Additionally, the strong T2 magnetic resonance (MR) signal and NIR photothermal transduction efficiency of the nanocomposites make it available for MR imaging and photothermal therapy (PTT). Altogether, this GO-Fe3O4@Au@Ag-MPBA-DOX can simultaneously fulfill the synergistic combination of cancer cell targeting, pH-sensitive drug release, SERS-traceable detection and MR imaging, endowing it great potential for SERS/MR imaging-guided efficient chemo-phototherapy on cancer treatment.

Menthol and Fatty Acid-Based Hydrophobic Deep Eutectic Solvents as Media for Enzyme Activation

This research aims to provide insights into the biological efficacy of a newly formed hydrophobic deep eutectic solvent (DES). A DES based on menthol was successfully synthesized with fatty acids. The DESs’ properties as enzyme activators were examined against a neat counterpart. The menthol:decanoic acid (1:1) combination showed improved thermal stability, strong catalytic activity, and reusability for up to four subsequent cycles under ideal conditions (pH 7.0, 40 °C for 2 h). The hydrophobic DES replaced hexane in ester synthesis, where RNL@DES5 showed better fatty acid conversion compared to neat RNL. This study demonstrated promising applications of hydrophobic DESs in non-aqueous organic reactions.

Au Nanorods on Carbon-Based Nanomaterials as Nanohybrid Substrates for High-Efficiency Dynamic Surface-Enhanced Raman Scattering

Gold nanorods (AuNRs) with different aspect ratios were prepared by the seed-mediated growth method and combined with three carbon-based nanomaterials of multiple dimensions (i.e., zero-dimensional (0D) carbon black (CB), one-dimensional (1D) carbon nanotubes (CNTs), and two-dimensional (2D) graphene oxide (GO)). The AuNR/carbon-based nanomaterial hybrids were utilized in dynamic surface-enhanced Raman scattering (D-SERS). First, cetyltrimethylammonium bromide (CTAB) was used to stabilize and coat the AuNRs, enabling them to be dispersed in water and conferring a positive charge to the surface. AuNR/carbon-based nanomaterial hybrids were then formed via electrostatic attraction with the negatively charged carbon-based nanomaterials. Subsequently, the AuNR/carbon-based nanomaterial hybrids were utilized as large-area and highly sensitive Raman spectroscopy substrates. The AuNR/GO hybrids afforded the best signal enhancement because the thickness of GO was less than 5 nm, which enabled the AuNRs adsorbed on GO to produce a good three-dimensional hotspot effect. The enhancement factor (EF) of the AuNR/GO hybrids for the dye molecule Rhodamine 6G (R6G) reached 1 × 10⁷, where the limit of detection (LOD) was 10^-8 M. The hybrids were further applied in D-SERS (detecting samples transitioning from the wet state to the dry state). During solvent evaporation, the system spontaneously formed many hotspots, which greatly enhanced the SERS signal. The final experimental results demonstrated that the AuNR/GO hybrids afforded the best D-SERS signal enhancement. The EF value for R6G reached 1.1 × 10⁸ after 27 min, with a limit of detection of 10^-9 M at 27 min. Therefore, the AuNR/GO nanohybrids have extremely high sensitivity as molecular sensing elements for SERS and are also very suitable for the rapid detection of single molecules in water quality and environmental management.

Flexible surface-enhanced Raman scatting substrates: recent advances in their principles, design strategies, diversified material selections and applications

Surface-enhanced Raman scattering (SERS) is widely used as a powerful analytical technology in cutting-edge areas such as food safety, biology, chemistry, and medical diagnosis, providing ultra-fast, ultra-sensitive, nondestructive characterization and achieving ultra-high detection sensitivity even down to the single-molecule level. Development of Raman spectroscopy is strongly dependent on high-performance SERS substrates, which have long evolved from the early days of rough metal electrodes to periodic nanopatterned arrays building on solid supporting substrates. For rigid SERS substrates, however, their applications are restricted by sophisticated pretreatments for detecting solid samples with non-planar surfaces. It is therefore essential to reassert the principles in constructing flexible SERS substrates. Herein, we comprehensively review the state-of-the-art in understanding, preparing and using flexible SERS. The basic mechanisms behind the flexible SERS are briefly outlined, typical design strategies are highlighted and diversified selection of materials in preparing flexible SERS substrates are reviewed. Then the recent achievements of various interdisciplinary applications based on flexible SERS substrates are summarized. Finally, the challenges and perspectives for future evolution of flexible SERS and their applications are demonstrated. We propose new research directions focused on stimulating the real potential of SERS as an advanced analytical technique for commercialization.

Plasmonic Metal Nanoparticles Hybridized with 2D Nanomaterials for SERS Detection: A Review

In SERS analysis, the specificity of molecular fingerprints is combined with potential single-molecule sensitivity so that is an attractive tool to detect molecules in trace amounts. Although several substrates have been widely used from early on, there are still some problems such as the difficulties to bind some molecules to the substrate. With the development of nanotechnology, an increasing interest has been focused on plasmonic metal nanoparticles hybridized with (2D) nanomaterials due to their unique properties. More frequently, the excellent properties of the hybrids compounds have been used to improve the drawbacks of the SERS platforms in order to create a system with outstanding properties. In this review, the physics and working principles of SERS will be provided along with the properties of differently shaped metal nanoparticles. After that, an overview on how the hybrid compounds can be engineered to obtain the SERS platform with unique properties will be given.

Direct Laser Writing of Copper Micropatterns from Deep Eutectic Solvents Using Pulsed near-IR Radiation

In this study, we developed a method for the fabrication of electrically conductive copper patterns of arbitrary topology and films on dielectric substrates, by improved laser-induced synthesis from deep eutectic solvents. A significant increase in the processing efficiency was achieved by acceptor substrate pretreatment, with the laser-induced microplasma technique, using auxiliary glass substrates and optional laser post-processing of the recorded structures; thus, the proposed approach offers a complete manufacturing cycle, utilizing a single, commercially available, pulsed Yb fiber laser system. The potential implications of the presented research are amplified by the observation of laser-induced periodic surface structures (LIPSSs) that may be useful for the further tuning of tracks' functional properties.

Long Spiky Au-Ag Nanostar Based Fiber Probe for Surface Enhanced Raman Spectroscopy

The detection performances of noble metal-based surface enhanced Raman spectroscopy (SERS) devices are determined by the compositions and geometries of the metal nanostructures, as well as the substrates. In the current study, long spiky Au-Ag alloy nanostars were synthesized, and both core diameters and spike lengths were controlled by Lauryl sulfobetaine concentrations (as the nanostructure growth skeleton). The long spiky star geometries were confirmed by transmission electron micrograph images. Elements energy dispersive spectrometer mapping confirmed that Au and Ag elements were inhomogeneously distributed in the nanostructures and demonstrated a higher Ag content at surface for potential better SERS performance. Selected synthesized spiky nanostars were uniformly assembled on multi-mode silica fiber for probe fabrication by silanization. The SERS performance were characterized using crystal violet (CV) and rhodamine 6G (R6G) as analyte molecules. The lowest detection limit could reach as low as 10^-8 M, with a 6.23 × 10⁶ enhancement factor, and the relationship between analyte concentrations and Raman intensities was linear for both CV and R6G, which indicated the potential qualitative and quantitative molecule detection applications. Moreover, the fiber probes also showed good reproducibility and stability in the ambient atmosphere.

The synergistic effects of coupling Au nanoparticles with an alkynyl Co(II) phthalocyanine on the detection of prostate specific antigen

Prostate specific antigen (PSA) aptasensors are fabricated using a novel asymmetrically substituted Co phthalocyanine (CoPc), gold nanoparticles (AuNPs) and PSA-specific antigen. The fabricated aptasensors are: GCE-AuNPs-Aptamer, GCE@CoPc-Aptamer and GCE-AuNPs@CoPc-Aptamer (GCE = glassy carbon electrode). The fabricated sensors are characterized at each modification step to monitor the changes occurring at the sensor surface. Concentration studies were carried out using differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS) to determine detection limits. All the fabricated aptasensors were found to be highly specific and selective but the GCE-AuNPs@CoPc-Aptamer nanoconjugate performed the best. The aptasensors were also tested in spiked serum samples and detection limits, as well as % recoveries were determined. The results obtained showed that the GCE-AuNPs@CoPc-Aptamer has the potential to be used for clinical studies as the results agree with those obtained for detection of PSA in buffer.

A recyclable graphene/Ag/TiO2 SERS substrate with high stability and reproducibility for detection of dye molecules

Magnetron sputtering combined with the wet chemical transfer of graphene successfully prepared a multilayer composite material and an efficient photocatalytic renewable SERS substrate. It has excellent photocatalytic activity against dye molecules.

Surface Enhanced Raman Spectroscopy: Applications in Agriculture and Food Safety

Recent global warming has resulted in shifting of weather patterns and led to intensification of natural disasters and upsurges in pests and diseases. As a result, global food systems are under pressure and need adjustments to meet the change—often by pesticides. Unfortunately, such agrochemicals are harmful for humans and the environment, and consequently need to be monitored. Traditional detection methods currently used are time consuming in terms of sample preparation, are high cost, and devices are typically not portable. Recently, Surface Enhanced Raman Scattering (SERS) has emerged as an attractive candidate for rapid, high sensitivity and high selectivity detection of contaminants relevant to the food industry and environmental monitoring. In this review, the principles of SERS as well as recent SERS substrate fabrication methods are first discussed. Following this, their development and applications for agrifood safety is reviewed, with focus on detection of dye molecules, melamine in food products, and the detection of different classes of pesticides such as organophosphate and neonicotinoids.

Tip-to-tip assembly of urchin-like Au nanostar at water-oil interface for surface-enhanced Raman spectroscopy detection

Au Nanostar (NS) monolayer as a surface enhanced Raman scattering (SERS) substrate has been synthesized by self-assembly at a water-oil interface. It is confirmed from the experiment and simulation results that the Au NS monolayer includes lots of "hot spots" at or between the tips of the Au NSs, enhancing the local electromagnetic fields and giving rise to strong SERS signals sequentially. The limit of detection is determined to be down to 4.2 × 10^-12 M for rhodamine 6G. Furthermore, the Au NS monolayer can detect multiple molecules, including thiabendazole, methylene blue, 4-mercaptobenzoic acid, and p-amino thiophenol, indicating that the SERS substrate composed of Au NS monolayer has potential applications in analytical chemistry, food safety, and environmental safety.

Large-area periodic arrays of gold nanostars derived from HEPES-, DMF-, and ascorbic-acid-driven syntheses

With arms radiating from a central core, gold nanostars represent a unique and fascinating class of nanomaterials from which extraordinary plasmonic properties are derived. Despite their relevance to sensing applications, methods for fabricating homogeneous populations of nanostars on large-area planar surfaces in truly periodic arrays is lacking. Herein, the fabrication of nanostar arrays is demonstrated through the formation of hexagonal patterns of near-hemispherical gold seeds and their subsequent exposure to a liquid-state chemical environment that is conducive to colloidal nanostar formation. Three different colloidal nanostar protocols were targeted where HEPES, DMF, and ascorbic acid represent a key reagent in their respective redox chemistries. Only the DMF-driven synthesis proved readily adaptable to the substrate-based platform but nanostar-like structures emerged from the other protocols when synthetic controls such as reaction kinetics, the addition of Ag⁺ ions, and pH adjustments were applied. Because the nanostars were derived from near-hemispherical seeds, they acquired a unique geometry that resembles a conventional nanostar that has been truncated near its midsection. Simulations of plasmonic properties of this geometry reveal that such structures can exhibit maximum near-field intensities that are as much as seven-times greater than the standard nanostar geometry, a finding that is corroborated by surface-enhanced Raman scattering (SERS) measurements showing large enhancement factors. The study adds nanostars to the library of nanostructure geometries that are amenable to large-area periodic arrays and provides a potential pathway for the nanofabrication of SERS substrates with even greater enhancements.

Mixed-Dimensional Heterostructure Material-Based SERS for Trace Level Identification of Breast Cancer-Derived Exosomes

Raman spectroscopy has capability for fingerprint molecular identification with high sensitivity if weak Raman scattering signal can be enhanced by several orders of magnitudes. Herein, we report a heterostructure-based surface-enhanced Raman spectroscopy (SERS) platform using 2D graphene oxide (GO) and 0D plasmonic gold nanostar (GNS), with capability of Raman enhancement factor (EF) in the range of ∼1010 via light-matter and matter-matter interactions. The current manuscript reveals huge Raman enhancement for heterostructure materials occurring via both electromagnetic enhancement mechanism though plasmonic GNS nanoparticle (EF ∼107) and chemical enhancement mechanism through 2D-GO material (EF ∼102). Finite-difference time-domain (FDTD) simulation data and experimental investigation indicate that GNS allows light to be concentrated into nanoscale "hotspots" formed on the heterostructure surface, which significantly enhanced Raman efficiency via a plasmon-exciton light coupling process. Notably, we have shown that mixed-dimensional heterostructure-based SERS can be used for tracking of cancer-derived exosomes from triple-negative breast cancer and HER2(+) breast cancer with a limit of detection (LOD) of 3.8 × 102 exosomes/mL for TNBC-derived exosomes and 4.4 × 102 exosomes/mL for HER2(+) breast cancer-derived exosomes.