ay in Introduction
The purpose of this thesis is to improve the already existing heat-transfer method biosensor setup. This first chapter starts of by sketching the background of the heat-transfer method. Next a concise description of the problems and disadvantages of this method is given. Furthermore these problems will be converted into objectives. The penultimate section of this chapter discusses the methods and materials that were used to achieve the established objectives. Lastly, a brief outline of this thesis is provided.
Background
A generally accepted definition of a biosensor, provided by Professor Anthony P. F. Turner, co-editor of “Biosensors: Fundamentals and Applications”, goes as follows:
“A biosensor is a device incorporating a biological sensing element either intimately connected or integrated within a transducer. The usual aim is to produce a digital electronic signal which is proportional to the concentration of a specific chemical or set of chemicals [1].”
Given the description above it is easy to understand why biosensors have become indispensable in a broad range of scientific domains including medicine, biology, pharmacology and food industries. Since these industries constantly endeavor to reduce costs, is expected that the demand for fast and effective biosensors will only continue to grow in the next years [2].
In order to answer to this demand, development of efficient readout techniques is more important than ever. However, the development of an efficient biosensor starts off with a sensitive and reliable biosensing element. While biosensing elements based on biological receptors provide high sensitivity and specificity toward their target, they can become unstable in challenging chemical and physical environments [3].
One robust yet efficient alternative towards biological receptors is the use of molecularly imprinted polymers (MIPs). Molecularly imprinting is a technique by which actual target molecules can be used to imprint cavities in a polymer. Since the shape of these cavities, also called binding sites, is complementary to the shape of the target molecules, the polymer exhibits high selectivity to the binding of these target molecules. Thus using this method, the potentially unstable natural biosensing receptors can be replaced by synthetic receptors that provide strong physical robustness, resistance to elevated temperatures and inertness towards acids [4].
Imagine a setup whereby a known volume of liquid containing target molecules is in direct contact with the MIP. In this setup, the number of target molecules bound to the MIP gives a direct indication of the concentration of target molecules in the liquid. In order to detect and quantify these target-receptor bindings, a broad range of different readout techniques have already been tested. Examples of these readout techniques are amperometry, potentiometry [5], [6] and optical methods which can detect changes in the MIP in terms of total charge and reflective index respectively.
At IMO-IMOMEC, a novel biosensing technique based on heat-transfer, the so called heat-transfer method (HTM), has been developed. This technique, presented in 2012, can use the change in thermal resistance (Rth) of a MIP to quantify the number of target-receptor bindings that occurred [7]. The change in thermal resistance takes place when target molecules bind to cavities in the MIP. As the imprinted cavities get filled with target molecules, heat transfer trough the MIP gets impeded, thus the thermal resistance of the MIP increases.
In order to monitor the heat-transfer trough the MIP the setup shown in Figure 1 is used. The sensor chip is connected to a copper bock. A PID controlled heating element will elevate the temperature of this copper block to a constant temperature above room temperature. The temperature of T1 is measured by a thermocouple. The temperature of the buffer solution, T2, is monitored by a second thermocouple. Given T1-T2 and the power (P) provided by the heating element, the thermal resistance (Rth) can be calculated using the formula (1) below. Thus at any moment during the measurement, Rth can be calculated. In case target-receptor bindings occur, an increase of Rth will be noticed. This technique has been successfully tested for the detection of L-nicotine, histamine and serotonin [8].
Rth= (T1-T2)/P
(1)
Figure 1- Schematic layout of the HTM setup [3]
Problem definition
Even though the HTM setup has shown promising results, and can be used for detecting a wide range of target molecules, it also comes with some disadvantages. While the copper block (Figure 1) does a good job of maintaining T1 at a constant temperature, its bulky dimensions cause slow reaction times of the system. This translates into long stabilization times and thus long measurement times that can vary between 45 minutes to 90 minutes per analyte. In addition to this, the bulky heatsink also makes it impossible to mass produce this setup in a profitable manner.
Another drawback is that the sensitivity of the setup is directly dependent on the accuracy of the thermocouples as well as the stability of the PID controller for the heating element. Especially this last element can be the cause of errors since different chip substrates may require different settings for the PID coefficients.
In order to overcome these drawbacks, researchers at IMO-IMOMEC have already come up with a new design (Figure 2). In this new setup, the copper heating element is reduced down to a resistive heater on PCB. Furthermore, this setup does no longer require thermocouples, since temperature information will be acquired by monitoring the electrical resistance using the 4-wire method.
To this new setup (Figure 2), multiple measurement principles can be applied in order obtain insight in the thermal resistance of the functional layer. Examples of these measurement principles are the constant resistance method (CRM), and the transient plane source (TPS) method. These measurement principles are further explained in chapter 5. At the start of this thesis, only the first of these methods has been explored, but the control system behind the sensor is in its infancy. The present PID controller is not able to maintain the sensor at constant resistance and thus no noteworthy measurement results have yet been obtained.
Figure 2- Prototype setup of the new biosensor
Objectives
The purpose of this thesis is to explore and test the different measurement principles mentioned at the end of the previous paragraph. Therefore a LabVIEW program needs to be designed in which these different measurement techniques can be selected and applied to the sensor. The user interface must be accessible to people in the lab and measurement data needs to be stored in a .csv file.
For the constant resistance method, a fast, yet stable control system is required. Similar to the original HTM setup, this control system will also be implemented in LabVIEW. One difficulty that arises here is the fact that the initial electrical resistance of the sensors can vary between 1Ω and 300Ω depending on the type of resistor that is used (paragraph 4.1).
In order to get insight in thermal resistance using the TPS method, sufficiently accurate curve fitting of measurement data needs to be done. First it was opted to do this curve fitting in LabVIEW during the measurement. Later was decided to do the data analysis after the measurement using MATLAB. In order for this data analyzation to be accessible by people in the lab, the MATLAB code will be implemented in an easy to use application.
Also hardware-wise work needs to be done. Since the working principle of the sensor is based on heat-transfer, influence of external temperature changes should be minimized. Also thermal capacity of the sensor itself needs to be taken into consideration therefore not only heaters on PCB will be tested, but also heating structures printed on pvc using conductive ink.
In order to determine if the sensor is actually able to detect differences in thermal resistivity at the sensor surface, a testing method needs to be chosen by which there is insight in the thermal resistance of the analyte before the measurement. This way the acquired data can be compared to the expected values. In order to compare the efficiency of different measurement techniques, this testing method also needs to have good reproducibility.
Methods and materials
The monitoring of the electrical resistance of the sensor will be executed suing the 4-wire method. In this method, two wires are used to source a current trough the sensor, while two other wires are used to measure the voltage across the sensor. This technique for measuring resistance has the great advantage that resistance of the leads and contacts is ignored. The hardware used for this purpose is a Keithley 2400 SourceMeter.
A LabVIEW application will be used to control the Keithley from a PC. Connection between these two will be made via GPIB protocol.
For the control system that needs to maintain the sensor at constant temperature, and thus also constant resistance, a feed-forward control loop will be implemented in LabVIEW. Feed-forward control overcomes the problem of having to change PID parameters depending on the initial electrical resistance of the sensor that is used. The choice for feed-forward control has been made after a literature study in adaptive PID control.
For the sensor hardware, different resistors will be tested. This group of sensors can divided into two main groups. Meanders on PCB and resistive heating structures that are printed on thin sheets of pvc using conductive ink. Taking into account the minuscule mass of the printed heaters compared to the heaters on PCB, it is expected for the printed heaters to exhibit faster system reaction times and thus provide higher sensitivity. In order to electrically insulate the sensors, in both cases Kapton tape will applied.
In order to test the accuracy of the sensor, and compare different measurement techniques, thermal conductivity measurements on milk will be executed. An experiment, executed using a similar sensing technique, has indicated there is a linear relation between the fat percentage of milk and its thermal conductivity [9]. Reproducing these measurements is not only interesting because of its good reproducibility, but also because at the same time, the linearity of the biosensor can be analyzed.
Outline
Chapter 2
Chapter two will succinctly go over a few key concepts that need to be understood in order to fully understand the working principle behind the sensor.
Chapter 3
Chapter 3 gives discusses the hardware and software concerning the original HTM setup.
Chapter 4
After having a look at the old setup in chapter 3, this chapter discusses the different key hardware components of the new setup. Different sensors will be reviewed, as well their sensor housing. Also the hardware used to perform the 4-wire measurement will be discussed.
Chapter 5
Chapter 5 gives the theoretical concepts behind the different measurement principles that are tested in this thesis. Techniques using constant resistance control as well as two different types of TPS methods will be explained.
Even though it has not been successfully applied in this thesis, also the concept behind the 3ω-method, will be illustrated.
Chapter 6
Chapter 6 reviews the work that has been done concerning software. This implies the LabVIEW program used to control the sensor, as well as the MATLAB application used for analyzing the TPS measurements.
Chapter 7
In chapter 7 the acquired results from different measurement techniques are compared and discussed.
Chapter 8
This chapter contains the conclusion and end result of this thesis. It summarizes what has been accomplished. To finish off with, unsolved issues are mentioned as well as suggestions on how to overcome these in order to further optimize the setup.
Literature study
Introductory concepts to thermal physics
Since the working principle of the sensor is based on heat transfer, it is of good interest to discuss some key principles of thermal physics
Heat
Heat is something we are confronted with every day. During a summer barbeque for instance, heat coming from the coals is responsible for the grilling of your meat. While you grab a cold drink afterwards, it is heat leaving your hand that is responsible for the cold sensation you experience. Therefore heat can be described as thermal energy in transit [10]. In addition to this definition, two important points need to be kept in mind concerning the concept of heat.
From experiments it is concluded that, when in contact, heat transfers from a body at higher temperature to a body at lower temperature. Heat transfer in the reverse direction cannot occur spontaneously.[10].
Another important thing to stress is the fact objects cannot contain a certain amount of heat. Hence the “in transit” part of the definition. The term heat can only be used in the context of energy being transferred due to a difference in temperature [10].
Heat is measured in joules (J). The rate of heating has units of watts (W), where W = 1Js-1, in words this means 1 watt = 1 joule per second
Thermal capacity
As explained in the previous section, it is not possible for
Thermal capacity, also called heat capacitity,
here…