# Recent Lipid Membrane-Based Biosensing Platforms
## 5 Practical Applications of Lipid Membrane Devices in Food, Environmental, and Clinical Analysis
### Applications of Graphene Based Devices  
A potentiometric urea lipid membrane-based mini-sensor on graphene has been reported recently [69]. A potentiometric urea device based on lipid film technology on graphene nanosheets has been constructed; a simplified set up of this biosensor is shown in (Figure 8). The main characteristics of this biosensor are excellent reproducibility, sensitivity, selectivity, reusability, and rapid response times; the slope of the electrode is ca. 70 mV/decade over the urea logarithmic concentration range, which can be determined from \(1\times 10^{-6}\) M to \(1\times 10^{-3}\) M.  
A nanosensor for naphthalene acetic acid (NAA) was reported in the literature and was based on polymerized stable-in-air lipid film devices, in which auxin-binding protein 1 receptor was co-deposited with the lipid film [71]. The sensor was evaluated in real samples of fruits and vegetables. Using a flow-through experimental set up, NAA was injected into the flowing 0.1 M KCl electrolyte solution and the flow was stopped. An ion current transient was obtained; the peak height was related to the concentration of NAA with mM detection limits and analysis times of ca. 5 min. Interference studies have shown no interferences from compounds normally found in foods and vegetables (e.g., lipids, proteins, ascorbic acid, etc.). The sensor has been used to determine NAA in fruits and vegetables with quite satisfactory results.  
A potentiometric carbofuran mini-sensor on graphene, in which lipid films were polymerized, was provided in the literature [70]. This device was applied to construct a carbofuran chemical sensor by  
Figure 8: Picture of the lipid film device on graphene mini-electrode, which was used for the potentiometric detection of urea (reprinted from ref. [83]).  
co-depositing resorcin[4]arene selective receptor. The detection limits were at the nM level, with times of response of 20 s. The device was constructed easily and showed excellent characteristics, such as good reproducibility, reusability, selectivity, long shelf life; the electrode slope was 59 mV/decade and carbofuran was logarithmically related in the range \(10^{-6}\) to \(10^{-3}\) M.  
The electrochemical interactions of cholera toxin with polymerized lipid films with incorporated ganglioside GM1 was reported in the literature [84]. An injection of cholera toxin in the flowing streams of a KCl 0.1 M carrier electrolyte provided a current signal. The peak height of the ion current signal was correlated with the concentration of cholera toxin in the sample solution, with detection limits of 0.06 uM.  
The response times and detection limits were improved using polymerized lipid membranes on graphene nanosheets (i.e., response times of 5 min, and detection limits of 1 nM) [72]. The construction of this sensor was easy and the sensor has shown good reproducibility, reusability, selectivity, long shelf life, and sensitivity of 60 mV/decade of toxin concentration. The method was applied in lake water samples.  
An electrochemical biosensor for the determination of saxitoxin based on graphene nanosheets with stable-in-air lipid membranes and immobilized anti- Saxitoxin (STX) was provided in the literature [73]. An excellent selectivity, sensitivity, and detection limits (1 nM) for the determination of saxitoxin with rapid response times (i.e., 5-20 min) were noticed. The sensor was easily constructed and had adequate storage stability with a slope of 60 mV/decade over saxitoxin concentration. The method was applied for the determination of STX in lake water and shellfish samples.