In the present work, tracing and Whatman papers were used as substrates to grow zinc oxide (ZnO) nanostructures. Cellulose-based substrates are cost-efficient, highly sensitive and environmentally friendly. ZnO nanostructures with hexagonal structure were synthesized by hydrothermal under microwave irradiation using an ultrafast approach, that is, a fixed synthesis time of 10 min. The effect of synthesis temperature on ZnO nanostructures was investigated from 70 to 130 °C. An Ultra Violet (UV)/Ozone treatment directly to the ZnO seed layer prior to microwave assisted synthesis revealed expressive differences regarding formation of the ZnO nanostructures. Structural characterization of the microwave synthesized materials was carried out by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The optical characterization has also been performed. The time resolved photocurrent of the devices in response to the UV turn on/off was investigated and it has been observed that the ZnO nanorod arrays grown on Whatman paper substrate present a responsivity 3 times superior than the ones grown on tracing paper. By using ZnO nanorods, the surface area-to-volume ratio will increase and will improve the sensor sensibility, making these types of materials good candidates for low cost and disposable UV sensors. The sensors were exposed to bending tests, proving their high stability, flexibility and adaptability to different surfaces.

Flexible electronics is a branch of electric fabrication that allows for increasingly ergonomic devices. However, its production still requires multi-step, expensive, and time-consuming processes. Laserdirect writing (LDW) is a clean and low-cost alternative technique for producing electrodes on flexible substrates with high resolution, without the need of masks or direct contact with the device. Laserinduced graphene (LIG), a particular type of LDW, is a technique in which, by the irradiation of polyimide (PI) and polyetherimide (PEI) films with a pulsed CO 2 infrared laser, a photothermal reaction occurs which leads to the formation of stacked graphene structures. LIG thus emerges as a rising substitute to produce graphene-based devices. This work consists of the development and optimization of flexible UV sensors with zinc oxide nanostructures as the active layer for photodetection, PI or PEI as the substrates, and the respective LIG as electrodes. The nanostructures with higher area-volume ratio, synthesized through a microwave-assisted hydrothermal method, were selected and deposited by drop-casting onto electrodes that in turn were optimized to enhance electrical properties by varying the laser parameters. The assembled sensors were able to successfully detect UV radiation with a responsivity of 92 and 2 nA W −1 for 1 V bias for the PI and PEI substrates, respectively. In addition, the PI sensor has shown to be capable of working under strain and to be stable after several hours of constant cyclic operation.

In this paper, we study the performance of a portable energy dispersive X-ray fluorescence spectrometer by making use of different filter configurations at the X-ray tube output. To fulfill this purpose, we systematically investigated how the detection limits (DL) can be improved by applying three different combinations of filters (Al, Cu and Al + Cu) between the X-ray tube and standard reference materials of different average-Z matrices.The obtained results were compared with the DL obtained with a benchtop system with triaxial geometry, considered the benchmark in detection of trace elements in a variety of sample matrices.The major differences occurred for the low-Z matrices, reflecting the increase of DL for elements with emission energies between 5 and 15 keV using filters. Regarding the high-Z matrices, the use of filters did not render an effective improvement in the DL but had a positive effect in diminishing sum peaks that could hinder the evaluation of some elements.

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