The Quartz Crystal Microbalance (QCM) is a highly sensitive tool for detecting changes in the mass of thin films on surfaces, offering insights into a wide range of physical and chemical processes. At the core of a QCM is a thin quartz disk with electrodes, typically made of gold or platinum, evaporated onto the crystal's surface. When an oscillating electric field is applied, a piezoelectric effect generates an acoustic wave through the quartz, and the frequency of this oscillation is inversely proportional to the mass of the crystal, including any thin layers deposited on it.
While the QCM is a sensitive detector for mass changes, it lacks inherent specificity. However, by depositing a thin sensing layer onto the QCM surface, specificity can be introduced, making it highly effective for applications such as immunosensing. A broad range of antibodies have been immobilized on QCM surfaces to detect various substances, including drugs (e.g., cocaine, quinine) and microorganisms (e.g., Candida albicans, Salmonella, and Escherichia coli). QCM has also been used to detect viruses, such as HIV, and has applications in DNA hybridization assays. The technique is also widely used for monitoring biofilm growth and microorganism detection in both water and clinical samples.
One challenge in QCM analysis of liquid samples is interference from other species adsorbing to the surface, similar to what occurs with Surface Plasmon Resonance (SPR) techniques. To address this, protective coatings or sample cleanup methods are used. The viscosity of liquids can also impact QCM response, requiring precise control of temperature and sample properties.
Microfluidics and Miniaturization
Microfluidics refers to systems that manipulate small volumes of fluids at micrometer scales. The development of microchip-based technologies has led to the creation of "lab-on-a-chip" devices that integrate multiple functions such as sample preparation, separation, and detection onto a single chip. These miniaturized systems offer significant advantages, including reduced sample volumes, faster analysis times, and the potential for point-of-care or on-site testing.
In microfluidics, electrochemical and optical detection methods are commonly used. Among electrochemical techniques, amperometry (current vs. applied potential) has gained the most traction, with many sensors employing electrodes made of carbon or metals like gold and platinum. This method has proven successful in detecting neurotransmitters, environmental pollutants, and in clinical assays. Potentiometric and conductometric methods are also used, with recent research focusing on contactless conductometric detection to reduce background noise and improve sensitivity.
Optical detection methods, such as absorbance, fluorescence, and chemiluminescence, are increasingly integrated into microfluidic devices for precise analysis. Additionally, mass spectrometry and infrared techniques are finding their place in miniaturized analytical devices.
Microelectrode Arrays
Microelectrode arrays (MEAs) offer several advantages over traditional macroelectrodes, such as improved sensitivity, smaller double-layer capacitances, and reduced solutional ohmic losses. These benefits make MEAs particularly useful in applications like disposable sensors (e.g., "dip-stick" devices), where electrodes can be placed directly into analyte solutions without causing unwanted fluctuations due to convection.
The fabrication of microelectrode arrays involves linking multiple microelectrodes together, and this can be done using photolithographic or laser ablation techniques. However, mass production of cost-effective and reproducible arrays has remained a challenge. Recently, a novel sonochemical ablation method for fabricating microelectrode arrays has been developed. This technique enables the mass production of microelectrodes with diameters ranging from 1 to 4 µm, offering an economically viable approach for disposable sensors.
