Nuria López-Ruiz

Smartphone-Based Simultaneous pH and Nitrite Colorimetric Determination for Paper Microfluidic Devices

In this work, an Android application for measurement of nitrite concentration and pH determination in combination with a low-cost paper-based microfluidic device is presented. The application uses seven sensing areas, containing the corresponding immobilized reagents, to produce selective color changes when a sample solution is placed in the sampling area. Under controlled conditions of light, using the flash of the smartphone as a light source, the image captured with the built-in camera is processed using a customized algorithm for multidetection of the colored sensing areas. The developed image-processing allows reducing the influence of the light source and the positioning of the microfluidic device in the picture. Then, the H (hue) and S (saturation) coordinates of the HSV color space are extracted and related to pH and nitrite concentration, respectively. A complete characterization of the sensing elements has been carried out as well as a full description of the image analysis for detection. The results show good use of a mobile phone as an analytical instrument. For the pH, the resolution obtained is 0.04 units of pH, 0.09 of accuracy, and a mean squared error of 0.167. With regard to nitrite, 0.51% at 4.0 mg L(-1) of resolution and 0.52 mg L(-1) as the limit of detection was achieved.

Recent developments in computer vision-based analytical chemistry: A tutorial review

Chemical analysis based on colour changes recorded with imaging devices is gaining increasing interest. This is due to its several significant advantages, such as simplicity of use, and the fact that it is easily combinable with portable and widely distributed imaging devices, resulting in friendly analytical procedures in many areas that demand out-of-lab applications for in situ and real-time monitoring. This tutorial review covers computer vision-based analytical (CVAC) procedures and systems from 2005 to 2015, a period of time when 87.5% of the papers on this topic were published. The background regarding colour spaces and recent analytical system architectures of interest in analytical chemistry is presented in the form of a tutorial. Moreover, issues regarding images, such as the influence of illuminants, and the most relevant techniques for processing and analysing digital images are addressed. Some of the most relevant applications are then detailed, highlighting their main characteristics. Finally, our opinion about future perspectives is discussed.

Fast prototyping of paper-based microfluidic devices by contact stamping using indelible ink

Here we present a fast and cheap prototyping technique for the realisation of paper-based microfluidic devices simply by using a stamp and indelible ink. The proposed mechanism involves contact stamping of indelible ink to laboratory filter paper using a PDMS stamp, which defines the microfluidic structure. It is a cleanroom and washing steps-free method which provides a reproducible method for the production of functional paper-based microfluidic devices in a single step in less than 10 s. The method is fully characterised and the concept has been applied, as a proof-of-principle, for the realisation of a low-cost colorimetric glucose sensor.

Wearable System for Monitoring of Oxygen Concentration in Breath Based on Optical Sensor

This paper reports the development of a compact, portable prototype for the determination of gaseous oxygen in breath. This system is fully wearable, it can be worn by the user as an armband where the electronics are placed, i.e., connected to a mouthpiece where the breath is sensed. The prototype is based on an optical luminescent sensor consisting of a stabilized porphyrin dye directly deposited onto a color detector. The sensor response, when it is optically excited, is quenched by the surrounding oxygen. The intensity of the generated luminescence is registered by means of a digital color detector whose output data correspond to the coordinates of the RGB color space. For this particular optical sensor, the red coordinate is directly related to the concentration of O2, and it is sent to a remote device such as a smartphone or tablet by a Bluetooth link. An application for Android operative system has also been developed for data visualization and analysis. Employing this novel system, a high resolution in the range of few parts per billion in the determination of oxygen can be achieved.

A compact optical instrument with artificial neural network for pH determination.

The aim of this work was the determination of pH with a sensor array-based optical portable instrument. This sensor array consists of eleven membranes with selective colour changes at different pH intervals. The method for the pH calculation is based on the implementation of artificial neural networks that use the responses of the membranes to generate a final pH value. A multi-objective algorithm was used to select the minimum number of sensing elements required to achieve an accurate pH determination from the neural network, and also to minimise the network size. This helps to minimise instrument and array development costs and save on microprocessor energy consumption. A set of artificial neural networks that fulfils these requirements is proposed using different combinations of the membranes in the sensor array, and is evaluated in terms of accuracy and reliability. In the end, the network including the response of the eleven membranes in the sensor was selected for validation in the instrument prototype because of its high accuracy. The performance of the instrument was evaluated by measuring the pH of a large set of real samples, showing that high precision can be obtained in the full range.

Flexible Passive near Field Communication Tag for Multigas Sensing

In this work we present a full-passive flexible multigas sensing tag for the determination of oxygen, carbon dioxide, ammonia, and relative humidity readable by a smartphone. This tag is based on near field communication (NFC) technology for energy harvesting and data transmission to a smartphone. The gas sensors show an optic response that is read through high-resolution digital color detectors. A white LED is used as the common optical excitation source for all the sensors. Only a reduced electronics with very low power consumption is required for the reading of the optical responses and data transmission to a remote user. An application for the Android operating system has been developed for the power supplying and data reception from the tag. The responses of the sensors have been calibrated and fitted to simple functions, allowing a fast prediction of the gases concentration. Cross-sensitivity has also been evaluated, finding that in most of the cases it is negligible or easily correctable using the rest of the readings. The election of the target gases has been due to their importance in the monitoring of modified atmosphere packaging. The resolutions and limits of detection measured are suitable for such kinds of applications.

Portable multispectral imaging system based on Raspberry Pi

Purpose In this work, the authors aim to present a compact low-cost and portable spectral imaging system for general purposes. The developed system provides information that can be used for a fast in situ identification and classification of samples based on the analysis of captured images. The connectivity of the instrument allows a deeper analysis of the images in an external computer. Design/methodology/approach The wavelength selection of the system is carried out by light multiplexing through a light-emitting diode panel where eight wavelengths covering the spectrum from ultraviolet (UV) to near-infrared region (NIR) have been included. The image sensor used is a red green blue – infrared (RGB-IR) micro-camera controlled by a Raspberry Pi board where a basic image processing algorithm has been programmed. It allows the visualization in an integrated display of the reflectance and the histogram of the images at each wavelength, including UV and NIRs. Findings The prototype has been tested by analyzing several samples in a variety of applications such as detection of damaged, over-ripe and sprayed fruit, classification of different type of plastic materials and determination of properties of water. Originality/value The designed system presents some advantages as being non-expensive and portable in comparison to other multispectral imaging systems. The low-cost and size of the camera module connected to the Raspberry Pi provides a compact instrument for general purposes.

Computer Vision-Based Portable System for Nitroaromatics Discrimination

A computer vision-based portable measurement system is presented in this report. The system is based on a compact reader unit composed of a microcamera and a Raspberry Pi board as control unit. This reader can acquire and process images of a sensor array formed by four nonselective sensing chemistries. Processing these array images it is possible to identify and quantify eight different nitroaromatic compounds (both explosives and related compounds) by using chromatic coordinates of a color space. The system is also capable of sending the obtained information after the processing by a WiFi link to a smartphone in order to present the analysis result to the final user. The identification and quantification algorithm programmed in the Raspberry board is easy and quick enough to allow real time analysis. Nitroaromatic compounds analyzed in the range of mg/L were picric acid, 2,4-dinitrotoluene (2,4-DNT), 1,3-dinitrobenzene (1,3-DNB), 3,5-dinitrobenzonitrile (3,5-DNBN), 2-chloro-3,5-dinitrobenzotrifluoride (2-C-3,5-DNBF), 1,3,5-trinitrobenzene (TNB), 2,4,6-trinitrotoluene (TNT), and tetryl (TT).