Raman spectroscopy is the tool of choice in the physicochemical investigation of carbon nanomaterials. However, Raman analysis of graphene oxide (GO) is lagging in comparison to the rich information gained in the case of carbon nanotubes and graphene. Here, we carried out a joint current sensing atomic force microscopy (CSAFM) and Raman spectroscopy investigation of laser-reduced GO. Reduced graphene oxide (rGO) was obtained under different laser powers in the range from 0.1 to 10 mW (532 nm). We compare the Raman spectra and the electrical conductivity at the nanoscale obtained by current sensing atomic force microscopy. Our analysis shows that three bands in the second-order region (2D, D + G, 2G), in the range from 2500 to 3200 cm⁻¹, are uniquely sensitive to the degree of reduction. Moreover, we found that the changes in peak area ratios A_D+G/A_D and A_2G/A_D show a direct correlation with the electrical resistance of rGO. We establish an optical micro-spectroscopy way to assess the degree of reduction in laser-reduced GO. These new insights provide a convenient and useful way to investigate the reduction of rGO from the fitting analysis of Raman spectra, becoming a useful tool in fundamental research and the development of rGO-based microdevices.

Electrochemical sensors play a significant role in detecting chemical ions, molecules, and pathogens in water and other applications. These sensors are sensitive, portable, fast, inexpensive, and suitable for online and in-situ measurements compared to other methods. They can provide the detection for any compound that can undergo certain transformations within a potential window. It enables applications in multiple ion detection, mainly since these sensors are primarily non-specific. In this paper, we provide a survey of electrochemical sensors for the detection of water contaminants, i.e., pesticides, nitrate, nitrite, phosphorus, water hardeners, disinfectant, and other emergent contaminants (phenol, estrogen, gallic acid etc.). We focus on the influence of surface modification of the working electrodes by carbon nanomaterials, metallic nanostructures, imprinted polymers and evaluate the corresponding sensing performance. Especially for pesticides, which are challenging and need special care, we highlight biosensors, such as enzymatic sensors, immunobiosensor, aptasensors, and biomimetic sensors. We discuss the sensors’ overall performance, especially concerning real-sample performance and the capability for actual field application.

Gallium selenide (GaSe) is a layered semiconductor and a well-known nonlinear optical crystal. The discovery of graphene has created a new vast research field focusing on two-dimensional materials. We report on the nonlinear optical properties of few-layer GaSe using multiphoton microscopy. Both second- and third-harmonic generation from few-layer GaSe flakes were observed. Unexpectedly, even the peak at the wavelength of 390 nm, corresponding to the fourth-harmonic generation or the sum frequency generation from third-harmonic generation and pump light, was detected during the spectral measurements in thin GaSe flakes.