# Perspectives in Liquid-Crystal-Aided Nanotechnology and Nanoscience
## 5 Biological Applications: Biosensors and Drug Delivery  
With increasingly improved medical techniques, it is expected that people will live longer. However, the current circumstances are not that promising. Due to the emerging problem of a \"graying society\" as well as the rapid spread of \"illnesses of affluence,\" chronic conditions such as overweight, hypertension, heart disease, diabetes, etc. are still severely threatening people's health. Therefore, the advancement of more compelling techniques for immune detection of different kinds of disease biomarkers is necessary. LCs, which are susceptible to weak external stimuli, such as chemistry, solvent vapour, humidity or surface forces, have received increased attention for use in biological sensing since [PERSON] and his co-workers initiated the research field of using LC materials as sensing elements for detecting biomolecules [426]. The key idea of this study is using a nematic LC film with a specific alignment, either homogeneous or homeotropic, at an interface with an aqueous phase. Under  
Figure 16: **(a)** TEM images of dried CNCs and the formation of a CLC phase and its coexistence with the isotropic phase in a CNC suspension (upper image). The typical CLC fingerprint appearance and the helical arrangement of nanorods are illustrated below. Reproduced with permission from [160; 400,421; 423]. Copyright (2010), (2012), (2014), (2016) Springer Nature. **(b)** Colour change observed when a dried CNC film is immersed in water and the corresponding spectrum. Reproduced with permission from [416]. Copyright (2013) Elsevier. **(c)** Nanoparticle templating of the CNC-derived helical structure. Reproduced with permission from [424; 425]. Copyright (2012) John Wiley and Sons and (2017) National Academy of Sciences. polarizing optical microscopy, the LCs will show a particular texture. By introducing some \"impurity\" molecules into the aqueous phase, the alignment of the LC molecules at the interface will be altered due to the binding between the biomolecules and the doped impurity, thus leading to a texture transition (Figure 17). This transition is easily seen by the naked eye. Such LC-based biosensors can localize biomolecules to the micrometre scale and the procedure can be executed under ambient light even without an external electric power source. All these advantages lend LC-based biosensors great potential for the next generation of high-sensitivity, low-cost and label-free bioassays. Up to now, various investigations of LC-based biosensors have been reported [427; 428; 429; 430; 431; 432]. Very recently, [PERSON] et al. reported a label-free protein (bovine serum albumin, BSA) quantitative detection by a dual-frequency LC (DFLC)-based biosensor [433]. The spectra of the dielectric properties of DFLC in the presence of BSA at varied concentrations were measured and analysed. They demonstrated that the difference in dielectric properties between the low- and high-frequency regimes was related to the concentration of BSA, which permitted a quantitative measurement of the BSA concentration. The simplicity and versatility of LC-based biosensors and their sensitivity is restricted by the size and amount of biomolecules, which may restrict their applications in the bioassay range of low concentrations and trace molecules. It is thus significant to seek signal-enhancement strategies for LC-based sensors to circumvent the problem. Several studies have demonstrated that the disruption of the LC orientation can be enhanced by introducing nanoparticles, thus leading to an amplified optical signal [434; 435; 436; 437; 438; 439; 440] (Figure 18a,b). A recent study by [PERSON] et al. demonstrated a nickel nanoparticle-assisted LC cell system that can be used to visualized the enzymatic activities between cholyglycine hydrolase (CGH) and cholyglycine (CG). The trace doping of nickel nanoparticles induced a uniform homeotropic LC alignment that could be disturbed by introducing CG to the binding-immobilized CGH surface.