A minimally invasive microsensor specially designed for simultaneous dissolved oxygen and pH biofilm profiling

A novel sensing device for simultaneous dissolved oxygen (DO) and pH monitoring specially designed for biofilm profiling is presented in this work. This device enabled the recording of instantaneous DO and pH dynamic profiles within biofilms, improving the tools available for the study and the characterization of biological systems. The microsensor consisted of two parallel arrays of microelectrodes. Microelectrodes used for DO sensing were bare gold electrodes, while microelectrodes used for pH sensing were platinum-based electrodes modified using electrodeposited iridium oxide. The device was fabricated with a polyimide (Kapton®) film of 127 µm as a substrate for minimizing the damage caused on the biofilm structure during its insertion. The electrodes were covered with a Nafion® layer to increase sensor stability and repeatability and to avoid electrode surface fouling. DO microelectrodes showed a linear response in the range 0-8 mg L˗1, a detection limit of 0.05 mg L-1, and a sensitivity of 2.06 nA L mg-1. pH electrodes showed a linear super-Nernstian response (74.2 ± 0.7 mV/pH unit) in a wide pH range (pH 4-9). The multi-analyte sensor array was validated in a flat plate bioreactor where simultaneous and instantaneous pH and DO profiles within a sulfide oxidizing biofilm were recorded. The electrodes spatial resolution, the monitoring sensitivity, and the minimally invasive features exhibited by the proposed microsensor improved biofilm monitoring performance, enabling the quantification of mass transfer resistances and the assessment of biological activity

Functionalized graphene sensors for real time monitoring fermentation processes:electrochemical and chemiresistive sensors

We developed a reference-less, chemiresistive, solid-state pH sensor to determine the acidification of the fermentation liquid in real time during the growth of Lactococcus lactis. One of the crucial findings of this work was that the ERGO-PA could not be used as such. It appeared that it was necessary to protect the sensor area with a Nafion coating to measure the pH in the fermentation broth. Most likely, the change in the concentration of redox-active components in the fermentation broth influences the conductivity of the ERGO-PA. Nafion formed a cation-selective membrane on top of the ERGO-PA allowing protons to diffuse to the selective layer of the sensor but not the redox-active components in the fermentation medium. We also reported a new approach to measure the dissolved oxygen concentration (DO) in a fermentation broth. The functionality of the sensor to measure DO was demonstrated during the growth of the obligate aerobic actinomycete Amycalotopsis methanolica in miniaturized 3D-printed bioreactors. For this oxygen-sensing application, the required modifications were obtained by doping hydrothermally reduced graphene oxide with nitrogen and boron atoms (N,B-HRGO). Further, these chemiresistive sensors are housed in the 3D printed bioreactor lid and used to measure pH, DO, and biomass in 3 ml fermentation broth. Additionally, the pH-sensor was equipped with a small heating element and a temperature sensor and that could be used for temperature control of the fermentation liquid. The setup was demonstrated to measure the pH, DO, temperature and biomass concentration in four parallel bioreactors

Electrochemical Microsensors for In Situ Monitoring of Chemical Compounds in Engineered and Natural Aquatic Systems

The adaption of needle-type electrochemical microsensor (or microelectrode) techniques to environmental science and engineering systems has transformed how we understand mass transport in biotic and abiotic processes. Their small tip diameter (5-20μm) makes them a unique experimental tool for direct measurements of analytes with high spatial and temporal resolutions, providing a quantitative analysis of flux, diffusion, and reaction rate at a microscale that cannot be obtained using conventional analytical tools. However, their applications have been primarily limited to understanding mass transport dynamics and kinetics in biofilms. With the advancement of sensor fabrication and utilization techniques, their potential applications can surpass conventional biofilm processes. In this dissertation, microsensors were utilized to elucidate mass transport and chemical reactions in multidisciplinary research areas including biological nutrient uptake, oily wastewater treatment, photocatalytic disinfection, and plant disease management, which have not yet explored using this emerging technology. The main objective of this work was to develop novel microsensors and use them for better understanding various natural and engineered aquatic systems. These include; 1) investigating localized photo-aeration and algal-bacterial symbiotic interaction in an advanced algal-bacterial biofilm process for nutrient removal from wastewater, 2) characterizing oil-in-water emulsions for better understanding bilge water emulsion stability, 3) evaluating sun-light driven photocatalytic reactions using a novel MoS2 nanofilm for water disinfection and microcystins-LR removal, 4) developing a zinc ion-selective microsensor and applying them for monitoring the transport of zinc in citrus trees, and 5) integrating heavy metal detection using anodic stripping voltammetry (ASV) in a microelectrode platform for plant applications. Overall, microsensors capable of measuring pH, oxidation-potential reduction (ORP), dissolved oxygen (DO), ammonia (NH3), hydrogen peroxide (H2O2), and zinc (Zn2+) were developed and applied to the systems described above to significantly contribute to a better understanding of interfacial transport mechanisms in various natural and engineered systems

Pediatric Cystic Fibrosis Sputum Can Be Chemically Dynamic, Anoxic, and Extremely Reduced Due to Hydrogen Sulfide Formation

Severe and persistent bacterial lung infections characterize cystic fibrosis (CF). While several studies have documented the microbial diversity within CF lung mucus, we know much less about the inorganic chemistry that constrains microbial metabolic processes and their distribution. We hypothesized that sputum is chemically heterogeneous both within and between patients. To test this, we measured microprofiles of oxygen and sulfide concentrations as well as pH and oxidation-reduction potentials in 48 sputum samples from 22 pediatric patients with CF. Inorganic ions were measured in 20 samples from 12 patients. In all cases, oxygen was depleted within the first few millimeters below the sputum-air interface. Apart from this steep oxycline, anoxia dominated the sputum environment. Different sputum samples exhibited a broad range of redox conditions, with either oxidizing (16 mV to 355 mV) or reducing (−300 to −107 mV) potentials. The majority of reduced samples contained hydrogen sulfide and had a low pH (2.9 to 6.5). Sulfide concentrations increased at a rate of 0.30 µM H_2S/min. Nitrous oxide was detected in only one sample that also contained sulfide. Microenvironmental variability was observed both within a single patient over time and between patients. Modeling oxygen dynamics within CF mucus plugs indicates that anoxic zones vary as a function of bacterial load and mucus thickness and can occupy a significant portion of the mucus volume. Thus, aerobic respiration accounts only partially for pathogen survival in CF sputum, motivating research to identify mechanisms of survival under conditions that span fluctuating redox states, including sulfidic environments

Fully Integrated Biochip Platforms for Advanced Healthcare

Recent advances in microelectronics and biosensors are enabling developments of innovative biochips for advanced healthcare by providing fully integrated platforms for continuous monitoring of a large set of human disease biomarkers. Continuous monitoring of several human metabolites can be addressed by using fully integrated and minimally invasive devices located in the sub-cutis, typically in the peritoneal region. This extends the techniques of continuous monitoring of glucose currently being pursued with diabetic patients. However, several issues have to be considered in order to succeed in developing fully integrated and minimally invasive implantable devices. These innovative devices require a high-degree of integration, minimal invasive surgery, long-term biocompatibility, security and privacy in data transmission, high reliability, high reproducibility, high specificity, low detection limit and high sensitivity. Recent advances in the field have already proposed possible solutions for several of these issues. The aim of the present paper is to present a broad spectrum of recent results and to propose future directions of development in order to obtain fully implantable systems for the continuous monitoring of the human metabolism in advanced healthcare applications

Development of Micro-Electrode Array Sensors for Electrochemical Detection of Dissolved Oxygen

Microelectrode array (MEA) dissolved-oxygen (DO) sensors were built and electrochemically tested in a solution of potassium ferricyanide. MEAs are becoming more popular as DO sensors because of their small size and capacity for simultaneous measurements with multiple recording sites. The ability to measure DO with multiple recording sites is useful for monitoring systems such as biofilm metabolism, which is an important factor in wastewater treatment. MEAs are beneficial because they are less destructive than individual microelectrodes that need to be moved through a sample to collect multiple measurements. The MEA used in the present work used gold electrodes, and a separate silver/silver chloride wire was used as the counter/reference electrode. The ferri/ferrocyanide redox couple is a reversible reaction with a well-known electrochemical behavior, making it a good way to test the electrochemical functionality of the MEA before using it to measure DO. Voltage was applied to the gold electrode in potassium ferricyanide solution, which initiated a redox reaction. The movement of electrons increases with the number of redox reactions occurring, meaning that measured current increases proportionally to the concentration of ions that get oxidized or reduced. The experiments were done inside a Faraday cage to minimize noise, and cyclic voltammograms were collected. There was a good linear response to the potassium ferricyanide, and the data was close to what was predicted by the Randles-Sevcik equation. The shape of the cyclic voltammograms appeared to have the characteristics of a reversible reaction, but the difference in voltage between the two peak currents was higher than expected. This may be due to an extra voltage drop at the counter electrode that is independent of the reactions, or high resistance due to inadequate electrolyte concentrations. Other possible interfering factors include adsorption at the electrodes and solution composition. The linear response of the sensor and its agreement with the Randles-Sevcik equation are promising signs that the sensor can measure differences in concentration, but a better understanding of what factors interfere with measurements is needed before applying the MEA to measuring DO concentrations