There has been an upsurge of green reductants for the preparation of graphene materials taking consideration of human health and the environment in recent years. In this paper, reduced graphene oxides (RGOs) were prepared by chemical reduction of graphene oxide (GO) with three green reductants, L-ascorbic acid (L-AA), D-glucose (D-GLC) and tea polyphenol (TP), and comparatively characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectra, Raman spectra and electrical conductivity analysis. Results showed that all these three reductants were effective to remove oxygen-containing functional groups in GO and restore the electrical conductivity of the obtained RGO. The RGO sample with L-ascorbic acid as a reductant and reduced with the existence of ammonia had the highest electrical conductivity (9.8 S·cm(-1)) among all the obtained RGO samples. The mechanisms regarding to the reduction of GO and the dispersion of RGO in water were also proposed. It is the good dispersibility of reduced graphene oxide in water that will facilitate its further use in composite materials and conductive ink.

/pdf/fabrication-and-characteristics-of-reduced-graphene-oxide-2yzvi4h61y.pdf

Fabrication and Characteristics of Reduced Graphene Oxide Produced with Different Green Reductants

Since the intriguing and inspiring discovery of graphene-enhanced Raman scattering (GERS) in 2010, graphene and graphene-based substrates have attracted significant attention in both theoretical research and application exploration of surface-enhanced Raman scattering (SERS); this makes graphene-involved SERS a burgeoning area in many scientific branches. The introduction of graphene not only overcomes some intrinsic drawbacks of SERS, but also has a great synergistic effect with traditional noble metal nanoparticle enhancement substrates and thus greatly improves the sensing performance of SERS and largely expands its application area. To better learn about the recent progress in graphene-based SERS substrates and shed light on future research, an overview of graphene and graphene-based SERS substrates is presented herein. In this review, the role played by graphene in graphene-based SERS substrates is first summarized, and then, the classification and preparation methods of graphene-based substrates are presented, which are helpful for understanding the structure–property relationship. Furthermore, the SERS applications of graphene-based substrates in biomedical areas, including biomolecule detection, bio-imaging, cancer diagnosis and therapy and drug delivery, are highlighted. Other applications in hot-topic areas such as food safety and environmental monitoring are also briefly discussed. Finally, challenges and perspectives on the deficiencies, future development and achievements are presented.

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

A green, one-step approach for the preparation of graphene/Ag nanocomposites (PE-HRG-Ag) via simultaneous reduction of both graphene oxide (GRO) and silver ions using Pulicaria glutinosa plant extract (PE) as reducing agent is reported. The plant extract functionalizes the surfaces of highly reduced graphene oxide (HRG) which helps in conjugating the Ag NPs to HRG. Increasing amounts of Ag precursor enhanced the density of Ag nanoparticles (NPs) on HRG. The preparation of PE-HRG-Ag nanocomposite is monitored by using ultraviolet–visible (UV-Vis) spectroscopy, powder X-ray diffraction (XRD), and energy dispersive X-ray (EDX). The as-prepared PE-HRG-Ag nanocomposities display excellent surface-enhanced Raman scattering (SERS) activity, and significantly increased the intensities of the Raman signal of graphene.

https://www.mdpi.com/1422-0067/16/1/1131/pdf

Pulicaria glutinosa extract: a toolbox to synthesize highly reduced graphene oxide-silver nanocomposites.

Plasmonic paper has received attention as a surface-enhanced Raman scattering (SERS) substrate due to several advantages. These include low cost, high rates of sample collection, high physical flexibility and relative ease of fabrication. Here, we report the development of a plasmonic paper based on a combination of graphene oxide (GO) and gold nanorods (AuNRs) on cellulose substrates using a simple layer-by-layer immersion method. Rhodamine 6G (R6G) was selected as the probe molecule to determine the SERS efficiency. The GO–AuNR plasmonic paper composite demonstrated superior signal intensity compared to that of the paper substrate coated only with AuNRs. The signal enhancement was due to synergism between the electromagnetic enhancement (EM) originating from the AuNRs and the chemical enhancement effect (CE) originating from GO. To optimize the fabrication process, the first step was accomplished by varying the GO concentration during the paper pretreatment step. This was followed by determining the number of immersion cycles required for optimal absorption of AuNRs. The highest SERS signals derived from R6G molecules indicated that the 2 mg mL−1 GO pretreatment condition followed by two immersion cycles into the AuNR solution proved to be the best condition for plasmonic paper fabrication. The enhancement factor (EF) of the plasmonic paper was calculated to be 4.56 × 107 and the limit of detection (LOD) for R6G analysis was as low as 0.1 nM. Moreover, the plasmonic paper was tested in the detection of Mitoxantrone (MTX), an anti-cancer drug. We found that the LOD for detection of MTX was on the order of 5 μM. This simple and cost effective plasmonic paper offers an alternative SERS substrate which has potential in trace analysis and drug monitoring.

Graphene oxide/gold nanorod plasmonic paper – a simple and cost-effective SERS substrate for anticancer drug analysis

The phase change enthalpy, thermal conductivity, thermal stability and thermal reliability of a novel reduced graphene oxide (r-GO) containing phase change material (PCM) r-GO/CaCl2·6H2O were investigated. The material was made by the aqueous dispersion of r-GO and calcium chloride dihydrate (CaCl2·2H2O) according to the mass ratio of CaCl2 and crystal water in CaCl2·6H2O. The thermal conductivity of the phase change material increased by ∼80% when using ∼0.018% (by weight) of r-GO with a ∼2.7% decrease of enthalpy (i.e., storage capacity), while using ∼0.018% of graphite led to an increase of thermal conductivity by ∼14% and a decrease of enthalpy by ∼5.6%. Additionally, the surface active agent for dispersing r-GO had the extra function of enhancing the system stability and reliability. The decomposing temperatures of r-GO/CaCl2·6H2O were higher than those of CaCl2·6H2O. After 100 cycles, the melting and crystallizing enthalpies of r-GO/CaCl2·6H2O decreased to 178.4 J g−1 and 150.7 J g−1 from 180.6 J g−1 and 153.7 J g−1, dropping by 1.2% and 2.0%, respectively, while for CaCl2·6H2O they decreased to 178.9 J g−1 and 147.8 J g−1 from 185.6 J g−1 and 161.8 J g−1, dropping by 3.7% and 8.7%, respectively. The thermal conductivity enhancement of CaCl2·6H2O with r-GO was markedly superior compared to that with graphite and other thermal conductive additives reported in previous literature, and the provided method (i.e., preparing aqueous dispersions of additives firstly and synthesizing hydrated salt PCMs with corresponding salts subsequently) was also applicable for other functional additives that cannot be directly dispersed well to modify the thermal properties of hydrated salt PCM systems.

/pdf/enhanced-thermal-conductivity-in-a-hydrated-salt-pcm-system-3vk03j61b5.pdf

Enhanced thermal conductivity in a hydrated salt PCM system with reduced graphene oxide aqueous dispersion

A free-standing and flexible silver nanoparticles-graphene (Ag NPs–GE) film with high surface-enhanced Raman scattering (SERS) sensitivity has been developed. First, the high stable and dispersive Ag NPs–GE composite was fabricated by in situ reduction using graphene oxide sheets as substrates and the hydrazine hydrate as a reducing agent in the solvothermal environment. Second, the Ag NPs–GE films can be fabricated through vacuum filtration of the Ag NPs–GE suspension. Transmission electron microscope (TEM) images show that the Ag NPs with an average size of 22 nm and a narrow particle size distribution are uniformly distributed on GE sheets. The coverage density of Ag NPs onto the surface of GE sheets can be successfully controlled by tuning the concentration of silver ion precursor. As-prepared Ag NPs–GE substrates have good SERS sensitivity due to the high SERS-active of Ag NPs. The enhancement factor for 1 × 10−6 M of rhodamine 6G molecules adsorbed onto the surface of substrate is about 2.7 × 105. The SERS activity toward crystal violet and Escherichia coli was also investigated.

Synthesis of flexible free-standing silver nanoparticles-graphene films and their surface-enhanced Raman scattering activity

Experimental study on the preparation, characterization and conductivity improvement of reduced graphene-oxide papers

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Synthesis of flexible free-standing silver nanoparticles-graphene films and their surface-enhanced Raman scattering activity

Experimental study on the preparation, characterization and conductivity improvement of reduced graphene-oxide papers