In this paper, we report the development of an oxygen sensor using a simple device made from a film of ZnO crystals dispersed over a paper surface. Due to the high porosity of this kind of device made from paper and ZnO, a large photoconductive effect is observed as result of the fast absorption and desorption of oxygen from the ZnO surface. To detect oxygen, we propose to use ultraviolet (UV) illumination over the photoconductive surface of the sensor, the applied UV light produces oxygen desorption causing current variations through the sensor that are proportional to the oxygen presence nearby the sensor surface. The sensor developed in this paper is attractive because it is remarkably easy to fabricate, employs only very low-cost materials for its construction, and shows a better sensitivity at low concentration of oxygen at partial vacuum environments.

With very simple methods, we fabricate photoelectrical devices using a regular paper as substrate; these sensors are able to detect ultraviolet, visible, and infrared radiation using different paper additives. The experiment we made to test detection of ultraviolet and visible light consists in irradiating light of different wavelengths over a device made of paper and semiconductor crystals; electrical current of these devices increases importantly when the bandgap energy of the semiconductor crystal is reached. To test the sensitivity to infrared, we use paper devices impregnated with salts and glycerol to increment its electrical conductivity due to the augmentation of ionic current carriers. We observe that the ionic conductivity on paper increases when the paper is irradiated by infrared light from hot objects; this phenomenon might be due to the molecular structure of cellulose that absorbs energy in the infrared part of the spectrum.

Herein we describe a method to measure turbidity in liquids by probing a graphic pattern by optical transmission. Data are collected by a video camera and a mathematical algorithm is used to correlate image loss with turbidity. We report high-resolution turbidity measurements of nanoparticle colloids to assess the proposed methodology. Measurements are differentially performed which helps reduce noise from external sources. Using Nephelometric Turbidity Units (NTU) standards we obtain detectable measurements in a broad range of NTU from 1.95 to 1000. By implementing the proposed methodology, we proposed a video-based inexpensive yet reliable turbidimetry instrument.