Flexible nano-cloth-like Ag cluster@rGO with ultrahigh SERS sensitivity for capture-optimization-detection due to effective molecule-substrate interactions

Surface-enhanced Raman scattering (SERS) is a rapid and promising detection technique for trace molecules. A central goal of research in this area is to achieve the highly sensitive detection of molecules built on a systematic understanding of enhancement mechanisms. Herein, we develop a Ag cluster@rGO composite nanostructure, which utilizes strong molecular adsorption to achieve ultrahigh SERS sensitivity. Ag clusters are prepared without additional reducing agents, leaving a low carbon footprint in the fabrication process. Finite-difference time-domain (FDTD) simulations show strong electromagnetic field enhancements generated at the edges and interstices of Ag clusters due to the specificity of their structure. Density Functional Theory (DFT) calculations show that the HOMO-LUMO energy gap value is significantly reduced when Ag cluster@rGO forms a composite system with the target molecule, which enables efficient charge transfer between the substrate and molecules, resulting in charge transfer enhancement. A detection limit of 10^-14 M using our substrate can be achieved for the environmental pollutant dye rhodamine 6G (Rh6G). The detection limits of bisphenol A (BPA) and its derivatives reach nanomolar levels with good signal stability. More importantly, we demonstrate the ability to rapidly screen BPA migration in Chinese Baijiu. Our SERS platform can be further developed for environmental pollution control and food safety.

Preparation of functional bamboo by combining nano-copper with hemicellulose and lignin under high voltage electric field (HVEF)

A simple and effective method for preparing functional bamboo by combining nano-copper with hemicellulose and lignin was proposed. The influences of HVEF treatment time and voltage on the reaction of nano-copper with hemicellulose and lignin were studied. The combination was characterized by scanning electron microscopy-X-ray energy dispersive spectrometer (ESEM-EDS), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The results indicated that the O/C ratio and OH content of the treated bamboo decreased, the number of CHO groups decreased, the number of CO groups increased, and the nano-copper interacted with the hemicellulose and lignin in the bamboo. Copper was presented in the treated bamboo in the form of Cu⁰, Cu⁺ and Cu²⁺. The concentration of copper increased with the increasing treatment time or voltage. The bamboo underwent the high-voltage electrostatic in situ impregnation copper treatment had significant antibacterial properties and excellent UV protection performance. Therefore, the in situ impregnation of nano-copper particles with an HVEF treatment is simple and effective to produce functionalized bamboo, and has broad application prospects.

Microplasma assisted synthesis of gold nanoparticle/graphene oxide nanocomposites and their potential application in SERS sensing

This is the first study on the deployment of direct current atmospheric pressure microplasma technique for the single step synthesis of gold nanoparticle/graphene oxide (AuNP/GO) nanocomposites. The nanocomposites were characterized using ultraviolet-visible spectroscopy (UV-vis), x-ray diffraction and x-ray photoelectron spectroscopy and their formation mechanisms have been discussed in detail. Our AuNP/GO nanocomposites are highly biocompatible and have demonstrated surface enhanced Raman scattering (SERS) properties as compared to pure AuNPs and pure GO. Their potential as SERS substrate has been further demonstrated using probe molecules (methylene blue) at different concentrations.

Recent progress on graphene-based substrates for surface-enhanced Raman scattering applications

Graphene-based SERS substrates are classified and introduced, and their applications in biosensing-related fields are reviewed.

Recent advances in the fabrication and structure-specific applications of graphene-based inorganic hybrid membranes

The preparation and applications of graphene (G)-based materials are attracting increasing interests due to their unique electronic, optical, magnetic, thermal, and mechanical properties. Compared to G-based hybrid and composite materials, G-based inorganic hybrid membrane (GIHM) offers enormous advantages ascribed to their facile synthesis, planar two-dimensional multilayer structure, high specific surface area, and mechanical stability, as well as their unique optical and mechanical properties. In this review, we report the recent advances in the technical fabrication and structure-specific applications of GIHMs with desirable thickness and compositions. In addition, the advantages and disadvantages of the methods utilized for creating GIHMs are discussed in detail. Finally, the potential applications and key challenges of GIHMs for future technical applications are mentioned.

One-step sonoelectrochemical fabrication of gold nanoparticle/carbon nanosheet hybrids for efficient surface-enhanced Raman scattering

A simple, fast, reproducible and efficient one-step fabrication method was successfully developed to prepare gold nanoparticle/carbon nanosheet (Au NP/CNS) hybrids by using sonoelectrochemistry. This method involved simultaneous generation of carbon nanosheets (CNSs) by oxidation of a graphite anode and generation of Au NPs by reduction of AuCl4(-) on the surface of the cathode. Then the Au NPs modified with poly(diallyl dimethyl ammonium chloride) were self-assembled on the surface of the CNS. A homemade sonoelectrochemical device that provided both high-intensity electric and ultrasonic fields was applied. The ability to obtain Au NPs with a controlled size and distribution on the surface of the CNS benefitted from the synergistic effect of the electric field and ultrasonic field. The Au NPs on the CNS surface exhibited distinctive and high-quality SERS activity. The enhancement factor of the developed substrate was 1.2 × 10(6) using 4-aminothiophenol as the probe molecule. The Au NP/CNS hybrid showed a great increase of Raman signals for aromatic molecules because of the high affinity of the CNS for aromatic molecules and the SERS activity of Au NPs. This SERS substrate also showed charge selectivity for cationic aromatic dyes, due to the negative charge on the surface of the CNS. Subsequently, the potential practical application of the SERS substrate was evaluated by quantitative analysis of adenine. The results suggest that Au NP/CNS materials as sensitive SERS-active substrates have great potential for detection of biomolecules.

A spongy graphene based bimorph actuator with ultra-large displacement towards biomimetic application

Bimorph actuators, consisting of two layers with asymmetric expansion and generating bending displacement, have been widely researched. Their actuation performances greatly rely on the difference of coefficients of thermal expansion (CTE) between the two material layers. Here, by introducing a spongy graphene (sG) paper with a large negative CTE as well as high electrical-to-thermal properties, an electromechanical sG/PDMS bimorph actuator is designed and fabricated, showing an ultra-large bending displacement output under low voltage stimulation (curvature of about 1.2 cm(-1) at 10 V for 3 s), a high displacement-to-length ratio (∼0.79), and vibration motion at AC voltage (up to 10 Hz), which is much larger and faster than that of the other electromechanical bimorph actuators. Based on the sG/PDMS bimorph serving as the "finger", a mechanical gripper is constructed to realize the fast manipulation of the objects under 0.1 Hz square wave voltage stimulation (0-8 V). The designed bimorph actuator coupled with ultra-large bending displacement, low driven voltage, and the ease of fabrication may open up substantial possibilities for the utilization of electromechanical actuators in practical biomimetic device applications.

Thin and transparent films of graphene/silver nanoparticles obtained at liquid-liquid interfaces: preparation, characterization and application as SERS substrates

We report here the synthesis and characterization of transparent and homogeneous thin films of reduced graphene oxide/silver nanoparticles (rGO/AgNPs) nanocomposites, starting from graphene oxide (GO) or reduced graphene oxide (rGO), directly obtained at a water/toluene liquid-liquid interface. Different films (obtained by varying the Ag/rGO or Ag/GO ratio) were prepared, deposited over glass or plastic substrates, and characterized by X-ray diffraction, UV-Vis and Raman spectroscopy, thermal analysis, transmission and scanning electron microscopy. Samples were evaluated as substrates for surface-enhanced Raman spectroscopy (SERS), using dilute solutions (1×10(-7) mol L(-1)) of a common probe molecule, 4-aminothiophenol (4-ATP). These materials exhibit significant high-quality SERS activity, and enhanced modes could be observed for 4-ATP, which suggested that charge transfer occurred between the Ag nanoparticles and 4-ATP molecules.

High-adhesive superhydrophobic 3D nanostructured silver films applied as sensitive, long-lived, reproducible and recyclable SERS substrates

Silver films with different morphologies were chemically deposited by controlling the bath composition. It is found that the wettability and surface enhanced Raman scattering (SERS) properties were closely connected with the surface morphology. Due to the perfect 3D morphology and the 3D electromagnetic field enhanced by three types of nanogaps distributed uniformly, the 3D microball/nanosheet (MN) silver film shows better SERS properties than those of 2D nanosheets (NSs) and nanoparticles (NPs). The MN silver film showed high adhesive superhydrophobic properties after an oxidation process without any functionalization. It can hold the liquid droplet and trace the target molecules in a rather small volume. The SERS properties of the oxidized MN substrate were enhanced remarkably compared to those of the freshly prepared substrate because of the concentrating effect of the superhydrophobicity. The as-prepared 3D MN silver substrate has also exhibited good performances in reproducibility and reutilization which makes it a promising substrate for molecule tracing.

Graphene oxide wrapped SERS tags: multifunctional platforms toward optical labeling, photothermal ablation of bacteria, and the monitoring of killing effect

As novel optical nanoprobes, surface-enhanced Raman scattering (SERS) tags have drawn growing interests in the application of biomedical imaging and phototherapies. Herein, we demonstrated a novel in situ synthesis strategy for GO wrapped gold nanocluster SERS tags by using a tris(2,2'-bipyridyl)ruthenium(II) chloride (Rubpy)/GO nanohybrid as a complex Raman reporter, inspired by the role of GO as an artificial receptor for various dyes. The introduction of GO in the synthesis procedure provided systematic solutions for controlling several key parameters of SERS tags, including reproducibility, sensitivity, and colloidal and signal stability. An additional interesting thermal-sensitive SERS property (SERS intensity decreased upon increasing the temperature) was also achieved due to the heat-induced release/redistribution of reporter molecules adsorbed on GO. Combining the synergic effect of these features, we further fabricated multifunctional, aldehyde group conjugated Au@Rubpy/GO SERS tags for optical labeling and photothermal ablation of bacteria. Sensitive Raman imaging of gram-positive (Staphylococcus aureus) and gram-negative (Escherichia coli) bacteria could be realized, and satisfactory photothermal killing efficacy for both bacteria was achieved. Our results also demonstrated the correlation among the SERS intensity decrease ratio, bacteria survival rate, and the terminal temperature of the tag-bacteria suspension, showing the possibility to use SERS assay to measure antibacterial response during the photothermal process using this tag.

Graphene oxide embedded sandwich nanostructures for enhanced Raman readout and their applications in pesticide monitoring

Analytical techniques based on surface-enhanced Raman scattering (SERS) suffer from a lack of reproducibility and reliability, thus hampering their practical applications. Herein, we have developed a SERS-active substrate based on a graphene oxide embedded sandwich nanostructure for ultrasensitive Raman signal readout. By using this novel Au@Ag NPs/GO/Au@Ag NPs sandwich nanostructure as a SERS substrate, the Raman signals of analytes were dramatically enhanced due to having plenty of hot spots on their surfaces and the unique structure of the graphene oxide sheets. These features make the sandwich nanostructured film an ideal SERS substrate to improve the sensitivity, reproducibility and reliability of the Raman readout. The sandwich nanostructure film can be applied to detect rhodamine-6G (R6G) with an enhancement factor (EF) of ∼7.0 × 10(7) and the pesticide thiram in commercial grape juice with a detection limit of as low as 0.1 μM (0.03 ppm), which is much lower than the maximal residue limit (MRL) of 7 ppm in fruit prescribed by the U.S. Environmental Protection Agency (EPA). The GO embedded sandwich nanostructure also has the ability to selectively detect dithiocarbamate compounds over other types of agricultural chemical. Furthermore, spiked tests show that the sandwich nanostructure can be used to monitor thiram in natural lake water and commercial grape juice without further treatment. In addition, the GO enhanced Raman spectroscopic technique offers potential practical applications for the on-site monitoring and assessment of pesticide residues in agricultural products and environments.

Graphene: a platform for surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) imparts Raman spectroscopy with the capability of detecting analytes at the single-molecule level, but the costs are also manifold, such as a loss of signal reproducibility. Despite remarkable steps having been taken, presently SERS still seems too young to shoulder analytical missions in various practical situations. By the virtue of its unique molecular structure and physical/chemical properties, the rise of graphene opens up a unique platform for SERS studies. In this review, the multi-role of graphene played in SERS is overviewed, including as a Raman probe, as a substrate, as an additive, and as a building block for a flat surface for SERS. Apart from versatile improvements of SERS performance towards applications, graphene-involved SERS studies are also expected to shed light on the fundamental mechanism of the SERS effect.