Investigation and Optimization of a Dual Electrode QCM Set-up for Sensing Biospecies in Liquids

Abstract

Introduction Quartz Crystal Microbalances (QCMs) are useful devices for chemical sensing, as they are easy to use and display high stability, rapid detection and easy portability. When mass is deposited on the gold electrode, a frequency decrease can be monitored. One application comprises detection of biological targets, such as proteins, viruses or bacteria [1]. Typically, these analytes are assessed in buffers, saline solutions or even real matrices, which are all comparably highly conductive. A change in conductivity may cause unwanted effects on the frequency shifts of a QCM if the set-up is not carefully designed and optimized. There is a strong influence of the ratio of the electrodes exposed to liquid samples (“front side”) and air (”air side”), respectively, on the extent of these frequency shifts [2,3]. Using a dual electrode set-up on one quartz plate gives the opportunity to use the second electrode as a reference, monitoring shifts in the frequency due to temperature changes or unspecific interactions of the analyte with the receptor layer [2,4].Herein, we report the design of a dual electrode QCM set-up, starting from an electrode design with a small ratio of liquid to air electrode size. By increasing the surface of the liquid electrode side, we were able to achieve small and even frequency responses for both – measurement and reference electrode – upon change of conductivity. Furthermore, the influence of the electrode surfaces on the liquid side were investigated for their performance. QCM set-up 10 MHz Quartz crystals, AT cut (35°15´) with a diameter of 13.8 mm were used. The “air side” electrodes were prepared by screen printing, the liquid side electrodes either by screen printing or sputtering. Before sputtering Au, we first deposited a thin chromium layer onto the substrate to ensure better adhesion for the subsequently sputtered gold layer. Method QCMs were equilibrated in water until recording stable frequency for both electrodes. To assess the effects of different sodium chloride solutions, the measurement cell was flushed with 1 mL of the respective solution. The measurements were performed under stop-flow conditions. After the frequency signals reached stable values, the next solution was added, or the solution was washed out by water. For comparing different electrode designs and surfaces, different concentrations of sodium chloride solutions were tested by comparing the resulting frequency shifts. Results and Conclusions Figure 1 shows two measurement curves recorded with two different dual-electrode designs (screen printed). The first design (Figure 1 A) has a ratio of liquid-to-air side electrodes of 1.25, which seems too small to effectively compensate for unwanted frequency shifts due to conductivity changes. The frequency shifts are not equal for both electrodes and one channel reaches a shift as high as -100 Hz when adding a 2.5 mM sodium chloride solution. However, the second design (Figure 1 B), having the liquid side fully metallized, reveals very substantial improvement over the first one: Adding different sodium chloride solutions leads to small frequency shifts of less than -5 Hz, which are very similar for both electrodes. Therefore, fully metallizing the QCM on the side exposed to liquid samples leads to superior results compared to the previous dual-electrode design comprising two individual electrode pairs.Furthermore, we compared two methods to deposit liquid electrodes, namely screen printing and sputtering. We carried out measurements using newly developed designs for both electrodes and measurement cell, keeping the principle of the liquid electrode side being fully metallized. Figure 2 compares the frequency shifts of both methods for different sodium chloride solutions: Screen printed electrodes show higher total signals and also higher standard deviations. The difference signal of the two electrodes (electrode 1 – electrode 2) was compared too. As can be seen in Figure 2 the frequency differences for different sodium chloride solutions are again higher for screen-printed devices, as are the corresponding standard deviations. These results suggest that screen-printing has weaknesses in terms of covering the whole quartz surface evenly and leads to comparably low reproducibility.On the other hand, sputtering is useful to coat the entire liquid electrode side with gold. This still allows for implementing a reference electrode that monitors frequency shifts due to density changes, while suppressing unwanted shifts caused by conductivity changes. For further work, design performance towards solutions with different viscosities is to be tested. Such an optimized dual electrode design will pave the way improving reproducibility and ruggedness during measurements in real matrices with an included reference electrode on a single quartz plate.