# Nanomaterials for Healthcare Biosensing Applications
## 4 Organic Nanomaterial Applications for Healthcare Biosensing
### Molecular Machines  
Molecular machines (MoMa), also known as nanomotors, are miniature devices which are able to move in a liquid phase with the help of an external power source or by self-propulsion. They can be defined as, \"an assembly of a discrete number of molecular components designed to perform mechanical-like movements (output) as a consequence of appropriate external stimuli (input)\" [306]. Molecular machines are at the cutting edge of scientific development with the 2016 Nobel Prize in chemistry being awarded to Jean-Pierre Sauvage, Sir James Fraser Stoddart, and Bernard (Ben) L. Feringa for their work on \"the design and synthesis of molecular machines\" [307]. Molecular machines functionalised with biomolecules are expediting the creation of ultramodern and meritorious biosensing systems. They can be conjugated with artificial molecularly imprinted receptors or biological molecules, such as antibodies or ODNs. Owing to their diminutive size and the vortex effect arising from their movement, motion-driven DNA nanomachines have led to ultrasensitive biodetection systems [231; 308]. Figure 13: Visualisation of the stoichiometric complex arising from the interaction of endotoxin with three optimum monomers, which were selected using molecular modelling [303]. Zhang et al. developed the first resonance Rayleigh scattering (RRS) strategy for uracil-DNA glycosylase (UDG), a base-excision repair enzyme critical to the repair of single-base mutations, by employing a label-free exonuclease III (Exo III) catalysed DNA nanomachine for dual amplification [309]. The nanomachine constituted a double stranded DNA (dsDNA) complex which would dissociate into two single strands that individually hybridise with hairpin probes (HP), triggering dual amplification catalysed by Exo III. The G-quadruplexes formed by the amplification reaction grew into continuous and elongated G-wires in the proximity of Mg\({}^{2+}\), providing an accentuated RRS intensity. A superior signal-to-noise ratio, along with high sensitivity, was obtained. A low LOD of \(1\times 10^{-5}\) U mL\({}^{-1}\) and a linear response between \(2.0\times 10^{-5}\) and \(0.4\) U mL\({}^{-1}\) was obtained. A qualitative analysis of this biosensing system was also performed successfully in HeLa cells lysate. Another approach using a DNA machine for UDG sensing based on the cleavage of uracil from HPs of the HP-G-quadruplex MoMa resulted in lower sensitivity [310]. In this case, the terminally exposed HPs with G-quadruplex sequences generate a G-quadruplex DNA structure induced by linked bridged hybridisation. The fluorescent intensity was significantly improved using thioflavin T (TfT). The LOD of \(7.8\times 10^{-3}\) U mL\({}^{-1}\) was thus two orders of magnitude higher than the previous study. A similar DNA machine was also reported for the detection on p53, the most common gene mutated in various cancers [311]. Cyclic nucleic acid strand displacement polymerisation (CNDP) was effectuated by p53, triggering the MoMa, which consists of an assistant template (AsT) and HP. The CNDP spurred a catalytic reaction mediated by DNAzyme. This reaction was responsible for the colorimetric detection. The p53 DNA, with the help of polymerase (POx) and nickase (NiOx), drove the MoMa one after the other through displacement cycles involving hybridisation and polymerization, creating a dramatic abundance of G-quadruplexes which mimicked HRP by conjugating with hemin, and were responsible for the \(2.2^{\prime}\)-azino-bis(3-ethylbenzothazoline-6-sulfonic acid) (ABTS) oxidation by H\({}_{2}\)O\({}_{2}\). This resulted in a colour change because of the presence of ABTS\({}^{\bullet+}\) oxidation product. The system provided a wide linear response range from \(10^{-14}\) to \(10^{-7}\) M with a limit of \(10\) fM.