## 4 Applications of Piezoelectric Materials in Analytical Chemistry
### Trace Metal Analysis  
[PERSON] and [PERSON] [127] created an electrogravimetry assay that uses piezoelectric crystals to determine trace metals in solution. The electrochemical cell's cathode was made of AT-cut quartz. After passing the known amount of current through the cell, the crystals were removed, washed, and dried. The concentration was calculated by utilizing the frequency shift caused by metal deposition. Cadmium solutions with concentrations ranging from 5.0 \(\times\) 10\({}^{-4}\) M to 5.0 \(\times\) 10\({}^{-8}\) M were investigated. Cadmium accurateness of this technique ranged from 0.42% at a large concentration to 8.7% at a small concentration. [PERSON] and [PERSON] [128] measured iodide in solution using another electro-gravimetric analysis using silver-covered piezoelectric crystals on a platinum-coated gold electrode. Iodide was electrodeposited at \(-\)0.05 Volts in a 10\({}^{-3}\) M potassium chloride (KCl) sample solution. The pH of the sample solution was adjusted to 9.8 with 10\({}^{-3}\) M sodium tetraborate (IH) caustic soda solution. This technique involves first running the reagent blank through the detection cell until it reaches a specific frequency. Iodide was present in the sample  
Figure 25: Label-free protein analysis by [PERSON] et al. reproduced with permission from [126], American Chemical Society, 2021.  
solution, which was then passed for 1 min (10\({}^{-6}\) M to 10\({}^{-5}\) M) or 10 min (10\({}^{-7}\) M to 10\({}^{-6}\) M). After each assay, \(-\)0.4 V electrolysis can be used to remove the iodide from the crystal. Lead was extracted from the solution using platinum-coated piezoelectric crystals. In another report, [PERSON] and [PERSON] [129] examined the stability of metal ions in solutions and the method for detecting iron (III) as a phosphate. For concentrations up to 2 mM, it has been demonstrated using standard piezoelectric crystals that the measured frequency change of metals in solution is proportional to the specific conductance, with variations owing to solution density and viscosity. The solution short-circuiting the crystal caused significant frequency changes at concentrations above 20 mM. Under these conditions metal ions i.e., Ni\({}^{2+}\), Mn\({}^{2+}\), Zn\({}^{2+}\), Co\({}^{2+}\), Cd\({}^{2+}\), Pb\({}^{2+}\), Ag\({}^{2+}\), and Cu\({}^{2+}\) was deposited on the electrodes. Changes in aluminum and iron frequencies have been attributed to salt adsorption. Another quartz plate was placed on the other side of the quartz plate, with just \"four thin hairs deposited on each of the sides of the'square' quartz plate\", separating the two. The plate was firmly bonded with epoxy resin. To avoid electrolysis, resin is also applied to the crystal holder's lead wire. It was discovered that the modified crystal responded linearly to lead (III) in the range of 1 \(\times\) 10\({}^{-5}\) M to 1 \(\times\) 10\({}^{-4}\) M. Interference experiments concerning an iron (III) solution at 5 \(\times\) 10\({}^{-5}\) M have shown that the 10-fold molarities can be harmful if deviations larger than 15% are attained. This occurs with aluminum, sulfide and thiosulphate, bismuth, and lead [106; 107; 108]. The contamination brought on by the existence of trace metals that could transmit through the in-filtrate water and soil is one of the main problems with mining operations and has a direct impact on the sustainability index for the environment. There are numerous methodologies that could be utilized to detect trace metals in the water and soil, as depicted in Figure 26, but the majority of them have limitations that make it impossible to apply them in real-world settings. There have been numerous attempts to develop portable sensor devices for regulating environmental trace metal concentrations. The transducer and the sensing elements are the two primary components of a traditional sensor for the precise detection of an analyte.