# Recent Lipid Membrane-Based Biosensing Platforms
## 5 Practical Applications of Lipid Membrane Devices in Food, Environmental, and Clinical Analysis
### Applications of the ZnO Nanoelectrode-Based Devices  
A potentiometric cholesterol device was constructed by immobilizing cholesterol oxidase into polymerized lipid membrane on ZnO nanowalls [75]. The enzyme was co-deposited into the lipid membrane prior to polymerization on the ZnO nanowalls surfaces, and provided a sensitive, selective, stable, and reproducible cholesterol device. The electrode slope was 57 mV/decade of cholesterol. No interferences were noticed by ascorbic acid, glucose, urea, proteins, and lipids. The present biosensor has shown biocompatibility and could be implanted in the human body.  
A uric acid electrochemical device was reported in the literature by immobilizing the enzyme uricase into polymerized lipid membranes on Zn nanowires [76]. The enzyme was co-deposited with the lipid membrane prior to polymerization on the surface of the electrode. The biosensor was sensitive, selective, stable, and reproducible. The presence of a cationic lipid in membranes has increased the electrode slope by two-fold. No interferences were observed by the presence of ascorbic acid, glucose, urea, proteins, and lipids.  
ZnO nanowires (NWs) were tailored for the immobilization of glucose oxidase in order to fabricate a glucose sensor [85]. The high specific surface area and isoelectric point provide the electrode efficient immobilization of high concentration of acidic enzymes. The apparent Michaelis constants were adjusted by tailoring the thickness of the GOD/ZnO nanowire layer and the enzyme loading in the nanowires. Through this route, linear region of sensitivity and reaction rates could be obtained. The long-term stability of this biosensor was high due to the inorganic ZnO NWs.  
Well-aligned ZnO nanowires were constructed on gold-coated plastic substrates using a low-temperature aqueous chemical growth method [86]. These arrays had 50-130 nm diameters and were applied to construct a urea biosensor using urease within the concentration range 0.1 to 100 mM with logarithmic response. The electrode slope was 52.8 mV/decade for 0.1-40 mM of urea, and response times were less than 4 s; this urea biosensor had excellent selectivity and reproducibility and showed no response to interferets such as ascorbic acid and uric acid, glucose, and K(+) and Na(+) ions.  
Well-aligned ZnO nanowires decorated with Pt nanoparticles (NPs) were recently used to construct a non-enzymatic glucose biosensor [87]. The use of Pt NPs decoration increased the sensitivity by 10-fold. The high specific surface area and isoelectric point (IEP) of ZnO has provided the electrode biocompatibility. A similar glucose biosensor on silicon NWs (ZnO/Si NWs) was also reported in the literature [88]. These nanowire nanocomposites have shown an excellent amperometric sensitivity to glucose (129 \(\upmu\)A\(\cdot\)mM\({}^{-1}\)), low detection limits (12 \(\upmu\)M), good stability and reproducibility, and selectivity in the presence of common interferents. Recent works applied a roll-to-roll flexographic printing technique to construct a three-electrode electrochemical unit that consisted of ZnO NWs [89]. This unit exhibited a sensitivity of 1.2 \(\pm\) 0.2 \(\upmu\)A\(\cdot\)mM\({}^{-1}\)\(\cdot\)cm\({}^{-2}\) and the calibration graph was linear over the glucose concentration between 0.1 to 3.6 mM.  
A ZnO NWs/Au electrode was constructed by immobilizing DNA for the fast detection of breast cancer 1 (BRCA1) gene [90]. This DNA biosensor was able to detect the target sequence in the concentration range between 10.0 and 100.0 \(\upmu\)M, with a detection limit of 3.32 \(\upmu\)M. A sensitive and selective label-free DNA ZnO NW device, which was based on a Schottky contacted, was also reported in the literature [91]. The performance of this device was greatly increased by the use of piezotronic effect [91].