Carbon Nanomaterials based on Graphene in (Electro-)chemical Sensors: Characterization, Modification and Application

The variability of graphene with its exceptional properties gives rise to improve material chemistry in various fields of applications. The development of graphene is still in the beginning and up to now only a few niche products have reached the market and are highlighted as well as the aim of this work in chapter 1. The goal of this thesis was the investigation of graphene in electrochemical sensor applications. It holds great promise in terms of miniaturization, improving sensitivity and developing new sensor concepts. Many approaches are already described for biosensor applications, often utilizing graphene in an amperometric detection scheme. Therefore, the impact of preparation technique on the detection of the model analyte H2O2 was investigated, applying different graphene materials as electrode material. Further, graphene was studied as tunable sensor material in gas sensing applications and how it can be customized for the room temperature detection of CO2. Chapter 2 summarizes the author’s publications and patents developed in the frame of this work. The perspectives of graphene in electrochemical sensors were investigated by the means of research performed in this field and are described in chapter 3. The most prominent preparation techniques and the application in biosensor and gas sensor technologies are discussed. Every method provides graphene materials of different characteristics, scalability and further usability. It was shown that defects in the ideal sp2 carbon lattice decide on the sensor performance, but also on device fabrication and appropriate functionalization. A higher quality of graphene can lead to more sensitivity and reliable device production in electrochemical biosensor technologies. In contrast, a defective structure can enhance the sensitivity and applicability in gas sensing applications, providing additional adsorption sites. In chapter 4, the experimental work, performed during this work, is described in detail. Chapter 5 comprises the results on graphene and graphene composite materials applied in the electrochemical detection of H2O2 and as tunable recognition element in gas sensors. In a first part, graphene materials derived by different preparation techniques were studied as electrode material. The electrochemical behavior of the different materials has been investigated as well as the feasibility in device fabrication was compared. It was shown, that the quality of the graphene has an enormous impact on the reductive amperometric detection of H2O2. A defective structure like in reduced graphene oxide leads to almost no significant improvement in signal enhancement compared to a standard carbon disc electrode. In contrast, fewer defects like in graphene prepared by chemical vapor deposition, resulted in a higher sensitivity, which is 50 times better compared to reduced graphene oxide. This technique was found to be most suitable for the production of highly sensitive electrodes to be further used in amperometric detection and development of biosensors. Whereas, a material of high quality is desired in electrochemical biosensor applications, defects are beneficial using graphene as transducer in a chemiresistive setup for the detection of gaseous analytes. In the next part of the work, reduced graphene oxide was demonstrated to be an applicable candidate for gas detection at moderate (85 °C) or even room temperature. Analyte gases like NO2, CH4 and H2 were detected due to fast changes in the electrical resistance at of 85 °C. To overcome the poor selectivity, the material was further altered with octadecylamine, metal nanoparticles such as Pd and Pt, and metal oxides such as MnO2, and TiO2. This changed the sensor response towards the studied gases and the different response patterns for six different materials allowed a clear discrimination of all test gases by pattern recognition based on principal component analysis. Based on the feasibility of this concept, a graphene-based sensor for the room temperature detection of CO2 was developed. Decoration of reduced graphene oxide with CuO nanoparticles led to an improved sensing performance for the target analyte. Different levels of metal oxide doping were applied by wet chemical and electrochemical preparation methods and the resulting composite materials were characterized. It was shown that a complete coverage obtained by wet chemical functionalization leads to highest sensitivity, comparable to a commercial CO2 sensor, which was also tested in the frame of this work. An array consisting of reduced graphene oxide and the composite with CuO nanoparticles was capable to differentiate CO2 from NO2, CO, H2 and CH4. This sensor material can lead to the development of miniaturized chemical sensors comprising high and adjustable sensitivity, which can be applied for monitoring air quality and ventilation management. The presented sensor concept based on customized graphene materials can be tailored for the versatile use in appropriate applications. One of the main challenges remains the reproducible large-scale production of graphene and functionalized graphene combined with reliable transfer techniques in terms of an industrial application. This problem has not been solved completely up to now. But the extensive research going on in this field will lead to a solution in near future and will help graphene to find its way to be integrated into many electrochemical sensor devices. Further studies should preferably aim for large scale production of the material and devices. The use of high quality graphene with a distinct introduction of defects and a better controlled way of functionalization may be a route to tune the material properties in favorable directions

Signal enhancement in amperometric peroxide detection by using graphene materials with low number of defects

Two-dimensional carbon nanomaterials ranging from single-layer graphene to defective structures such as chemically reduced graphene oxide were studied with respect to their use in electrodes and sensors. Their electrochemical properties and utility in terms of fabrication of sensing devices are compared. Specifically, the electrodes have been applied to reductive amperometric determination of hydrogen peroxide. Low-defect graphene (SG) was obtained through mechanical exfoliation of natural graphite, while higher-defect graphenes were produced by chemical vapor deposition (CVDG) and by chemical oxidation of graphite and subsequent reduction (rGO). The carbonaceous materials were mainly characterized by Raman microscopy. They were applied as electrode material and the electrochemical behavior was investigated by chronocoulometry, cyclic voltammetry, electrochemical impedance spectroscopy and amperometry and compared to a carbon disc electrode. It is shown that the quality of the graphene has an enormous impact on the amperometric performance. The use of carbon materials with many defects (like rGO) does not result in a significant improvement in signal compared to a plain carbon disc electrode. The sensitivity is 173 mA center dot M-1 center dot cm(-2) in case of using CVDG which is about 50 times better than that of a plain carbon disc electrode and about 7 times better than that of rGO. The limit of detection for hydrogen peroxide is 15.1 mu M (at a working potential of -0.3 V vs SCE) for CVDG. It is concluded that the application of two-dimensional carbon nanomaterials offers large perspectives in amperometric detection systems due to electrocatalytic effects that result in highly sensitive detection

Development of Graphene Based Inks for Deposition via Inkjet Printing for Sensing Application

In this work we investigate and optimize graphene based inks to achieve a stable and well-controllable jetting behavior using a DoD (Drop on Demand) inject printer which has all the required characteristics of a tool for mass production

A photonic crystal based sensing scheme for acetylcholine and acetylcholinesterase inhibitors

We present a new scheme for sensing biomolecules by combining an enzyme hydrogel with a photonic crystal hydrogel layer that responds to ionic strength and pH changes. We demonstrate this unique combination by successfully detecting acetylcholine (ACh) and acetylcholinesterase (AChE) inhibitors. Specifically, the sandwich assembly is composed of layers of photonic crystals and a polyacrylamide hydrogel functionalized with AChE. The photonic crystal film has a red color and turns dark purple within 2–6 minutes of the enzymatic reaction upon analyte addition. This 3D photonic crystal sensor responds to acetylcholine in the 1 nM to 10 μM concentration range (which includes the relevant range of ACh concentrations in human body fluids). Michaelis–Menten kinetics of the enzyme were determined which correlated well with literature data demonstrating the uninhibited reactivity of the immobilized enzyme. Furthermore, the presence of the acetylcholinesterase inhibitor neostigmine at concentrations as low as 1 fM was demonstrated, which is even below the necessary detection limit for clinical diagnostics. We suggest that this novel concept will find its application in clinical diagnostics, for pesticide and nerve agent detection

Reduced graphene oxide and graphene composite materials for improved gas sensing at low temperature

Reduced graphene oxide (rGO) was investigated as a material for use in chemiresistive gas sensors. The carbon nanomaterial was transferred onto a silicon wafer with interdigital gold electrodes. Spin coating turned out to be the most reliable transfer technique, resulting in consistent rGO layers of reproducible quality. Fast changes in the electrical resistance at a low operating temperature of 85 °C could be detected for the gases NO2, CH4 and H2. Especially upon adsorption of NO2 the high signal changes allowed a minimum detection of 0.3 ppm (S/N = 3). To overcome the poor selectivity, rGO was chemically functionalized with octadecylamine, or modified by doping with metal nanoparticles such as Pd and Pt, and also metal oxides such as MnO2, and TiO2. The different response patterns for six different materials allowed the discrimination of all of the test gases by pattern recognition based on principal component analysis